As far as eGO and MMO's go, from what I've seen, it's mostly sticking with World of Warcraft.
RuneScape use to be another supported MMO (Browser-based, too) but it was removed a little while ago due to inactivity/lack of support. Other then that, it's a really fun game (And it's free-to-play, while having some perks with membership subscriptions and other money-needed-packages)
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Rift I've played a little bit of, but personally don't like it. I agree with Diggles that it isn't a WoW Clone, let alone anywhere near it. Somewhat same-design, sure..but the...'feeling' of the game, if that's the correct way to put it, is just off.
GW2, or any of the Guild Wars Series, I haven't played or bought so no comment there, other then based off reviews/previews, it looks like a fun game/series.
Elder Scrolls Online, from what I've heard, has released it's beta (there's a couple posts/threads around the forums here with the link to website), and I must say it looks epic. Saw a couple youtube videos on it too from the game designers discussing their area of the game, such as PvP, the concept, etc. Really looking forward to this game!
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There's most likely more out there, but these, IMO, are the more popular games.