Defense Grid was a cheap throw in game I bought during a Steam sale for something like $2.50 but it ended up being one of my favorite games of that year. It is a really good tower defense game with personality. The company announced a Defense Grid 2 kickstarter which will be awesome if it is made. I kicked in some money of course.
One of the rewards is a copy of the original game and DLC. I already have them all so I am giving away the Steam codes for them if any tagged member doesn't have the game and would like to try it just send me a PM.