Defense Grid, a game created at Hidden Path Entertainment (Where I work as a game designer) is currently on save via the something called "The Potato Sack"
You can buy the pack and get a bunch of great indie games at 75% off, or you can buy Defense Grid individually for 50% off.
Defense Grid was made well before I got involved with Hidden Path, so I can honestly give you a non biased heads up that this game is awesome! It is without a doubt the most polished and well thought out tower defense game I've ever played. You will really be missing out if you don't take advantage of this sale, just sayin'.
Besides, there are some neat things going on with this indie pack if you take a look around at the game news.