I'm actually doing a weekly review series on the topic of Free-to-Play MMO games, so that's actually right up your alley. The pacing might be a bit slow for you, but if you'd like to read it anyway, send me a PM.
For the instant advice, it depends entirely on what you're in for, given the crazy amount of variety in games like these.
For the standard MMO-style game, there are almost too many to list. (MMOHut [http://mmohut.com/gamelist] and OnRPG [http://www.onrpg.com/MMO/Free-MMORPG] have good lists for that, though.) As far as what you want out of it, there are a few reliable standbys.
Soul of the Ultimate Nation is a solid choice. Runes of Magic (as suggested above) is almost picturesque WoW in terms of gameplay and interface. If WoW is your thing, this is free and almost the same experience. Allods Online is also reliable for that. There are a lot in this genre, so it's hard to advise one over the other. For a slightly different experience, Perfect World International is a gorgeous game that looks amazing on max specs, and changes enough in-game to be different.
If you're looking to break out of the mold a little, try something like League of Legends (made by the same guys behind DotA). It takes some getting used to, being an RTS-like game as just a single unit. There's a lot of waiting involved, and the community isn't very inviting, but the gameplay is fun assuming it's your sort of thing.
If not that, try Sword of the New World (also called Granado Espada), which gives the player a party of three instead of one character, which takes some micromanagement, but is pretty cool because of that. Atlantica Online has a turn-based combat system, which is very unique, but a little slow if you're not a fan of turn-based games.
D&D Online, formerly a subscription game, is solid. Although I wouldn't recommend trying it without a bit of background with D&D systems, and a familiarity with the rulesets of pen-and-paper games.
There's also Ace Online, which is a fighter-pilot title. It's Korean, meaning there's a lot of grind to be found, but is a genuinely active and new speed for games of the type. Remember to quest, as that's much better experience than standard monster-grinding.
If you're looking for something a little more Korean or anime-inspired, there are several highlights. Fiesta Online is the most generally well-recommended. Grand Fantasia as well. Or, if you're a fan of the Shin Megami Tensei series, try MegaTen. FlyFF (Fly for Free) is a cute and creative game, which has flight and a combo system. It can get a little repetitive, as most games of its type, but it's good to try if you enjoy that sort of thing. Ragnarok Online (the Valkyrie server is free) and Secret of the Solstice are good standbys for that genre of 2D fantasy game. Also very grindy.
The platformer titles are the ones that really shine in this category. MapleStory, Wonderking, or LaTale are all very good for that. Each has it's own perks and flaws, but none of them break anything. (Avoid Grand Chase Online, as that one isn't very good.) If you're into that kind of thing, but prefer 3D, try Dragonica Online (of which there is an Escapist guild. Feel free to PM me for that as well).
If you're not into any of the above, GunZ: The Duel and S4 League are both good for the "high speed" third-person-shooter genre. Which you'll like depends on how you like your atmosphere and speed settings. I personally prefer S4. If you want a bit less speed, try Battlefield Heroes. Exteel is highly recommended as well (though I can't verify, it does not run on Win7 or Vista). For a more traditional FPS: Alliance of Valiant Arms, Crossfire, and Combat Arms are all great for it.
For racers, there's TrackMania Nations Forever (also available on Steam), Project Torque, and Drift City. All three are good for different reasons. Project Torque is the most realistic of them all, but has an unreasonable amount of grind to upgrade cars and parts. TrackMania Nations Forever is more of an arcade-style racer, which is as much physics driven as nearly any Source game, and takes a bit of getting used to. Drift City is cel-shaded and absurd, but fun assuming you don't mind off-the-wall racers.
I'd also recommend trying Tales Runner or Project Powder.
There are also many Kart-style racing games, none of which are all that great. If you like combat in racing, I recommend UpShift Strike Racer. It really is the only one worth mentioning in the genre.
Um... I've already gone on at a stupidly absurd length. Is there anything you're specifically looking for?