I was asked to put together a list of free games to try, so I went over my archive...
* Alien Swarm: free multiplayer game from folks that worked on Left4Dead.
* Eversion: Mario meets Braid
* Hedgewars: multiplayer puzzle-strategy; huge international fanbase.
* Mario Paint Composer: for anyone that played Mario Paint when they were a kid.
* OpenTTD: simulate roads, rail, and airplanes; I actually got in trouble for playing this too late once.
* Privates: maybe your teenager wants to kill Herpes instead of catch it?
* Scorched 3D: tank war; single or multiplayer.
* Catan Online: also the official website for "Settlers of Catan"
* Desktop Tower Defense: a "spiritual successor" to Rampart
* FreeCiv.net: online Civilization clone
* Kingdom of Loathing: community-driven RPG; their unofficial radio station is awesome.
* Pirates Love Daises: comical HTML5 tower-defense
* Urban Dead: multiplayer survivor vs zombie RPG; you will often end up playing as both.