I found a new tower defense game via JIG, Warzone Tower Defense. It has a standard (but nice) selection of upgradable towers, and it's an open area (like the Desktop Tower Defense games) instead of pre-defined paths (like most every other tower defense game). As the waves go on and the enemies get tougher, you're forced to build mazes (see the image), which I find kind of tedious. Now, you're forced to build mazes in Desktop Tower Defense, too, but at least that ends. Warzone Tower Defense doesn't end, apparently. It just keeps going and going until you die. Now, this is obviously interesting to some people, as their forum is full of people bragging about the levels they reached (before dying or having their browser crash), sometimes with pictures showing the various complicated mazes they've built.
So, the game is definitely fun for a while, but I really do prefer my tower defense games to be winnable, even if it takes a really long time.