In my spare time, I've written various games in JavaScript, which means you can play them right in your web browser. You don't even need Flash or Java! They do depend on having a good browser, meaning SeaMonkey 1.0+ or Firefox 1.5+. Opera might work for some of them, and Internet Explorer probably won't work well for most.
Some of them are pretty decent, and some of them are awfully lame. Some of them I wrote just to see if I could, and some I wrote to actually produce an interesting result. Some will make your computer slow while you're playing them.
You might also want to take a look at my JS toys.
Nibbles - the classic worm game that came with QBasic
Missile Command - defend your cities against the commies' ICBMs!
Robots - escape from the insane killer robots
Sidescroller - Super Mario Brothers! (really incomplete)
The Older Scripts 5: Tomorrow Breeze - A role-playing / adventure game, the sequel to Bethesda's The Elder Scrolls IV: Oblivion (MAY CRASH YOUR BROWSER)