Have you heard of Play This Thing!? (Yes, there is a ! in the title) It comes to us courtesy of Greg Costikyan, your friend and mine, who says, “It’ll feature a game a day–free games, interactive fiction, mods, and weird stuff like alternative reality and ‘big urban’ games, as well, of course, as independent games.” Check it out.
As Nicodemus mentioned long ago, you can also find a lot of free flash games at Kongregate. It also has built in badges and things so you can feel like you are leveling and getting pretty shinies as you enjoy timewasters.