This weekend, Robot Entertainment (makers of such fine video games as Orcs Must Die!) are celebrating their 4th anniversary as a company with a big ol' sale of their products. On the company's official store, you can purchase the Steam versions of Orcs Must Die!, Orcs Must Die! 2 and Hero Academy, as well as some of the DLC for those titles at 75% off of their normal prices.
I do strongly recommend Orcs Must Die! 2, which wound up being a big improvement over the original (which was already quite enjoyable) and $6.24 for the game and its three booster packs should make this an easy decision for anyone who hasn't tried it out yet. The sale only runs through the weekend, though, so don't dally.