I'm particularly emphasising free ones, since I don't want to pay for something I might not like, particularly by subscription. For anyone else who might want the MMORPG experience without having to fork out for it, here are my experiences.
The ones I'm playing now
Talisman Online - Intended to resemble Chinese myth. Easy to play, but quite challenging. It has a few elements that it set it apart, such as the talisman system (wherein you use energy from kills/quests to upgrade certain items) and the master/apprentice system (which didn't work out in the end, because my master became inactive when I was at 28, effectively screwing me over at the last minute). www.talismanonline.com/
Shaiya - More effort in the visual presentation than many free MMOs. Easy to learn, but there are some translation issues (inconsistent monster names, for instance). Guilds seem easier to find and join in this than in some other games, which makes a difference. Lacks the additional unique (or at least distinctive) features of games like Talisman, but is very solid nonetheless, and the relative variety of character routes (for a free game) gives replay incentive. I've been told it's very much like World of Warcraft, but as I always say, "like WoW but free can only be good." One major criticism at the moment is for the last couple of weeks, there have been lots of server problems. Hopefully they won't last. shaiya.aeriagames.com/
Conquer Online - This was the first one I played, and consequently it has defined my experience of MMORPGs. Everything I play undergoes comparison to Conquer. It's not typical, though; very grind-heavy, and big on PvP. Easy to pick up and put down, but lacks depth. Unlike almost all other MMORPGs, Conquer doesn't really do quests. 1GB. co.91.com/
Nexus: The Kingdom of the Winds - Seems to be an MMORPG done in the style of old-school top-down RPGs. That alone was enough to intrigue me, and the tiny size of the client helped. Sadly, it didn't start very well; the keyboard-based control is a little awkward, and combat seems clumsy. Still, I've barely done anything with it yet - it's too early to pass judgement. 192MB. www.nexustk.com/index.html
The ones I've tried and dropped
Tales of Pirates - Anything with pirates is worth a try. Initially, the lighthearted cartoony approach put me off, but that actually turned out to be one of its best features. Easy to dip into, and its reluctance to take itself too seriously makes even grinding for levels less of a headache than in many other games. Unfortunately, the lightweight feel, apparent lack of depth, and sometimes twitchy controls mean that I ended up barely playing this after the first couple of weeks. Not a bad game, but lacks...gravitas. 2GB. top.igg.com/
Zu Online - Very pretty, and recommended by an acquaintance as one of the best free MMOs she's played, but my experience was that the game explained itself very badly and the interface was glitchy as hell (with the added irritation that you have to click slightly below whatever you're aiming at). 2GB. zu.igg.com/
Age of Armor - Ok, I haven't actually dropped this one, but I've given up on it because it's unusable. I've made three attempts to download it, and in addition to the download taking twenty-four hours my laptop has told me every time that the folder I downloaded was empty. Oh well. 1.3GB. aoa.us.woniu.com/index.html
Voyage Century - Unlike Tales of Pirates, this seems to be a pirate-based MMO that takes itself seriously. Unfortunately, it explains itself extremely badly. The first time I played, I managed to miss the entire tutorial, simply because the game doesn't make it clear what you're meant to do, and my wild guess was wrong. I was willing to press on, particularly in light of the inclusion of ship-to-ship combat, but for some reason it just stopped working, so I uninstalled it. 1.3GB. vc.igg.com/main.php
Ryzom - This looks interesting and really well done. The way the website describes it, it sounds like quite an ambitious project for a free game. And that's where it falls down - since I first looked at this, it's become a pay-to-play. You can download a 1.5-ish GB demo version, but when I tried it, it just didn't run for some reason Shame. This may have been great. 7GB. www.ryzom.com/en/index.html
Requiem: Bloodymare - This intrigued me by being 'the first true horror MMORPG'. It's very pretty, with good atmosphere and some intriguing ideas, such as more powerful monsters appearing in the wee hours of the morning (game time), forcing you to travel in parties or make sure you spend your nights in towns. Unfortunately, some aspects of the game don't work all that well (you have to memorise where you picked up quests, because neither the map nor the coordinates system will tell you) while some have clearly not been playtested (to hit an enemy you have to always be facing it, which makes combat very niggly when the enemy moves around a lot, as most do). Worse yet, the game lags almost continuously; research suggests I had it worse than most, but almost all players get it quite a lot. Perhaps worst of all, though, is the video error. A lot of people have to reinstall some video drivers because the game doesn't recognise them if they're already installed - which would be bad enough once, but it seems to happen every couple of updates. Grrr. Potentially very good, but fundamentally broken. www.playrequiem.com/default.aspx
La Tale - This was recommended to me as a fun 2D side-scroller. Having played it, I can confirm that it's certainly a 2D side-scroller (it plays like an 8-bit platformer with 16-bit graphics and some RPG trappings). It might also be fun. Sadly, it works so rarely that I never really had chance to find out. Eight times out of ten, it froze my entire computer, despite not being very resource-intensive; when it did work, it occasionally closed itself mid-play. So my verdict on this game is a frustrating one: it might be good or it might not - its technical problems make it almost impossible to play. latale.ogplanet.com/main.og
Fly F.F. - While investigating Rappelz (see below), I saw some remarks to the effect that Rappelz is just a prettier version of Flyff. I didn't play for long, but it was pretty forgettable. It's all done competently enough, but there's nothing engaging about it. Nothing in the game drew me in. Perhaps if I had a fondness for cartoony cuteness I might feel differently, but I just found it somewhat grating. If you want cartooniness, I'd suggest trying Tales of Pirates over this. It doesn't help matters that any time you enter a town, the game lags horrendously. 'Unremarkable' is probably the best way of describing this. Having said that, there is apparently a whole thing about flight later on in the game. It's a shame I never got far enough to check that out, but it just bored me. 669MB. flyff.gpotato.com/
The ones I'd like to try
Rappelz - Someone on the Shaiya Facebook group mentioned this, so I took a look. So far it's hard to tell what this may or may not have over other games. Still, the client is quite small for such a game, so I might give it a go. 1GB. rappelz.gpotato.com/
Zero Online - The only free science fiction MMORPG I've found. It's made and run by the Conquer team, so I expect it to be simple and grind-filled. Not filling me with joy, but worth a go - if only because SF MMORPGs seem to be so uncommon. 2GB. zo.91.com/index/
9 Dragons - Now this is one I'm really interested in. It's come highly recommended by the same person who recommended Zu (so I'm not taking it on trust). Shows some promise - not least because it's made by well-establish game company type of thing, Acclaim. No idea of client size. 9dragons.acclaim.com/
Maple Story - Apparently quite famous and, like the tragically unplayable La Tale (see 'Dropped'), a 2D side-scrolling MMORPG. For the few minutes that La Tale did actually work it seemed like it might be quite fun, so I'd like to give Maple Story, its more reputable counterpart, a spin and see how it goes. 1.5GB. www.maplestory.com/
Sword of the New World: Granado Espada - A very nice-looking MMORPG set in a fictitious parallel to the Spanish settlement of America. Though the main thing that draws me to it is the setting, it purports to have several unusual features, including control of up to three characters simultaneously. No more solo questing for the unsociable players among us! Bizarrely, an online translation service tells me 'Granado Espada' means 'Pomegranate Sword'... Unfortunately, the client is huge, so I might not get round to trying it. 4GB. swordofthenewworld.gamersfirst.com/index.php
Runes of Magic - This is the one that seems to be basically WoW. Unlike Shaiya, it already has various side-tasks (crafting and whatnot), and the classes etc seem to be pretty much the standard WoW-esque selection. Like Sword of the New World, the client's pretty big. I'd have to say, if I'm only going to download one of these two huge games, it'll be the more inventive Sword. Still, I'm curious. 4GB.
www.runesofmagic.com/en/index.html
Neo Steam - An ad for this popped up while I was looking for something else. At first it seemed identical to many other MMORPGs. After a few minutes, though, I noticed that it does seem to deliver (or at least it claims to deliver) on the 'steam' part of the title. Something about seige engines and constructing things. Hmm.
neosteam.gamigo.com/
Very few of these games are larger than 2GB, so not ridiculously large. They generally don't take too long to download either (except Shaiya, which took a good three hours). Nexus, though I haven't tried it yet, is apparently a microscopic 192MB!
I have 100GB to play with, two is nothing. Can't expect them to take up no space, you fool. 
