The evolution of the web as a gaming platform seems to be coming along lately. With games often serving as pioneers into new frontiers, I believe this is merely signifying the trend of software products transforming into services instead of commodities.
Anyway, what I mean to say is, I've been spending quite some time with web-based games lately. Mostly it's the fantastic and funny web-based parody RPG Kingdom of Loathing, which I've mentioned before. After a brief respite, I'm getting into this game again, having started dabbling with PvP, playing (or trying to play) the market, and going after the quests I haven't finished yet.
Another online game I found lately is the totally addictive time-waster gridlock, apparently a port of an existing puzzle game. It's moderately challenging; it took me two days (not continuously, of course) to finish the 40 levels.
I think it's also great that all three of these games work well with Firefox, without me having to install any silly extensions. (I'm not particularly fond of slow-loading flash games...)