Revenge of the Titans can be pre-ordered now for $13.37, though it's due to go up to $27.72 when it launches tomorrow for Windows, Mac and Linux. Having pre-ordered myself, I heartily recommend it. If my word's not good enough, though, you can download a demo of the latest version, which is presently the release candidate.
Casting players in a more active role than most TDs, Revenge of the Titans requires you to manually collect resources and reload weapons. When the forces of Titan are beating on your door, it becomes jolly good hectic fun. There's also a tech tree which spans the whole fifty-level campaign, letting you save for and unlock new toys as you see fit.