Tetris is the king of small addictive games. I could play it forever.
And Pacman. Ahhhhhhh, just all the old school games are awesome time wasters. But I hate Space Invasion.
There's a series of games called the Coffee Break games or something like that, basically a bunch of small addictive games designed to be played in a coffee break. Lots of really awesome ones, they're worth checking out. I think most of them are on the Under-dogs.
If you have a few links, give them! :D
Ahah, the Tower Defence from Warcraft 3. It is a pretty could map. This flash game is a good replica.