Flash and Shockwave Fun
The slogan on this site reads "A new addictive flash game every weekday, or your money back"--and they don't appear to be kidding. For instance, one recent game du jour was a charming time management title, Sushi Go Round, which challenges you to assemble various sushi orders for customers at a sushi-boat bar.
Games are categorized by type (Action, Classics, Puzzle, and so on), and there's a list of the most played games if you're the monkey-see, monkey-do type of casual gamer. You'll have to play a lot of games to run out of things to do here.
Shockwave.com has a mixed bag of downloadable and free Web-based games, for both single and multiple players. Many of the Web-based games are versions of popular desktop games: I tried out Sandlot Games' Cake Mania but found it painfully slow, even on my fast office connection, compared with the PC game (which Shockwave.com repeatedly offers as a $20 download).
Other games are just fine, though. A colleague confided that the Shockwave.com version of Bounce Out was her guilty pleasure, and you may find others here too.