Defenders Quest looks good - namedropping it for next month.
Defenders Quest is amazing and this is coming from a guy who doesn't like tower defense games. Well worth your time.
Just throwing this out there but you can buy Defender's Quest from the developer for about $5 (66% off the normal price.) The sale only lasts 2 more days I think. You can buy it off Steam for the same price, but buying it from the dev gives them more cash and gives you a DRM-free copy of the game (WITH a Steam key!.)
http://www.defendersquest.com/
I've only played the first few levels, but it's a decent TD game. The writing is fairly amateurish, but I still enjoyed it;D The mechanics (stats & whatnot) are all on display so it caters a bit more to the hardcore TD fan. Overall, the developer's love of the genre really shines through and I think that puts the game over the top.