Man I need some kind of TD tag. With my past forays into stand-alone titles like Pixeljunk Monsters, Defense Grid: The Awakening, and of course the original Warcraft III TD maps, it's maybe my most-played game type.
Plants vs. Zombies has been my most recent pickup. For $9.99 on Steam, I think it's certainly worth it. While not strictly a tower defense game, it has enough of a strategy feel to push all my buttons.
Graphically, the game is simple. Low res, but workable. Thankfully, the cartoony feel of the plants and zombies allow me to overcome my deep-seated fear of the undead and play the game, even at night!
The gameplay is varied, if a little shallow. There are a huge amount of towers, from pea shooters to watermelon catapults, wall-nuts (get it?) and spiky hedges. Night-time modes and aquatic hazards add a little complexity. But it's never difficult to see what you need to do, although there are a few strategies beyond just building a ton of crap to get in the way of zombies.
There are also endless varieties of mini-games, puzzle modes, and the main adventure mode. Heck, there's even a zen garden where you can grow all sorts of little plants.
Definitely getting my money's worth out of this title. If you enjoy strategy games, it's worth checking out.