Tech Cult blog has lovingly compiled 150 of the best Flash games available on the Web. If you have, oh, say a week off, then you can probably play all of the games listed here:
Link - Thanks Daniel Scocco!
The game on the left is insanely addictive Desktop Tower Defense, which is probably the most popular Flash game ever (previously discussed on Neatorama here, before it got famous, of course! | Desktop Defense Strategy)
Infectonator is great, too!
http://www.flashyourgame.com/infectonator-world-dominator/
not a fan of towerdefence games though