There are a few questionable ones today... I'm still not sure why Counter-Strike: GO is a paid game, given all the money it makes through weapon skins. But as usual, there are some ridiculously good games for ridiculously low prices.
Chivalry: Medieval Warfare | $US2.49
This game is hilarious fun. I'm not sure what the player populations are like online nowadays, but even if it's scant, it's an ideal game for LANs. Duels between two players are good enough in their own right, and the combat system is deep enough to make that interesting, but where it excels is in the chaos of big battles. The crunching sound of being killed by heavy weapons will never leave you.
Battlefield: Bad Company 2 | $US4.39
A triple-A shooter for around $5, yet again. Makes me feel sorry for all the indie FPS games that really have to intelligently pick their window of release these days. You might get lucky with player populations, too, given this game's habit of going on sale.
Age of Mythology | $US7.49
Dat classic. Don't be fooled by the hydras and minotaurs, this is one of Ensemble's best. I really can't say if there are lots of people playing online or not -- but play the vanilla version first, before you dabble with the expansion. Then make your own mind up as to whether it jumped the shark with the Atlanteans.
What are your picks out of the sales at the moment?