Some indie stuff:
Freeform tower defense: Harvest: Massive Encounter
standard tower defense: Defence Grid: The Awakening
Top down shooter with rpg elements: Crimsonland
Music/Rhythm/wierdness: Audiosurf
also Shadowgrounds and Shadowgrounds: Survivor are really good, you can pick them up pretty cheaply.
Not indie, but if you havent played Rome: Total War and like strategy games I recommend it, and you can pick it up for around $10 these days.