The Apocalyptic Game About Penguins!
I found out about this one while reading a Games for Windows magazine because I got bored with my yearbook. It's seriously a really well done 2D mouse clicky shooter. Nothing that hasn't really been done to death already, but as far as freeware shooters go, this one has polish and fun built right in. Seems pretty challenging and killing massive amounts of penguins with your weapons is damn satisfying.
So if you have time to kill and a hatred for flightless Eudyptes, I recommend this.
Oh and if you downloaded it and liked it because of me, you owe me your senior yearbook.