Since it seems that everyone is posting their favorite flavor, here's mine: http://spectacleapp.com/. Open source and gets you hooked in minutes. I can't imagine using OS X without it.
+1. I've been using spectacle with great success. I previously tied the demo for Sizeup, which is a little more polished, but cost $20 at the time. The only thing that Spectacle is missing is the ability to move windows between monitors. I recently switched to a single 27", so I haven't been missing this feature. I'm definitely going to check out Slate — looks cool.