Here's a remarkable new tower defense game called Strategy Defense 3.
In addition to playing pretty darn well, it has some very cool features and beautiful graphics.
Multiple maps and difficulty levels, more variety of objects than i've ever seen -- amazing stuff.
I love this genre because it really embodies the concept of "casual" games and it's great to see how different designers innovate.
Strategy Defense does a few really cool new things. First of all, there is very little tower building or upgrading, most of what you do is just select which new unit to build when you get new money, making this one of the simpler games to actually play. But there is still a lot of strategy involved.
The variety of levels also definitely requires some strategic thinking -- some of the levels require only long distance attacking for example.
A really wonderful addition to the genre and a great first tower defense game for those who have never played one before.