Me, since I don't have all that many people to play my various RTS's with, I play several different browser-based games - most because of invites from friends. They consume almost all of my free time - what they don't use is consumed by EVE Online or whatever other random thing I happen to be doing at the time. My current list is (I'll provide links for them, in case anyone's interested):
Star Wars Combine: a Star Wars simulation. Probably one of the more accurate sims I've found recently, with a full economy in effect. No combat yet (which, I think, is a good thing, since I would get destroyed rather fast, I think

), but production has been recently released. There's still a long way to go, but SWC is still cool in my opinion.
RuneScape: Probably one of the most well-known MMORPG's out there. I don't play it that much (since I started playing EVE Online, actually), but it's still not all that bad, in my opinion.
Astro Empires: A strategic space war game. One of my newer interests, I've become rather adept at defense in it. Though some things can take some time (a month to build a Leviathan...

), I find it fairly interesting, mainly through watching the politics going on in the forums (that can get quite heated at times).
Cyber Nations: An online nation simulation game. I grew proud of my nation...though now it stands a shadow of what it once was, nearly annihlated by a massive, one-sided war (my alliance vs. ten others...we're outnumbered 8-1 in terms of nation strength and nations arrayed against us. Down to ~800k from ~11mil nation strength before the war...). GATO barely exists anymore...But those that remain stand firm.
These are the main ones. What do you think of browser-based games, and what (if any) do you play?