Vectoid Tower Defense. (Link swiped from
the_zaniak who clearly wants me to never get anything done again, ever.) It's an awesome and very addictive Flash game :)
From Peter's post:
In short: you're given a map, and some money to set up turrets. Press a button, and the vectoids appear. For every one that your turrets kill, you get more money to buy more turrets, and the vectoids slowly get tougher and more valuable.
Give it a go. In no time, you'll learn to hate the yellows, love the third red turrets, and watch as your lives trickle away. Hint: the cheap blues are the most useful turrets in the game.
This game has ated my brain. Send help!