By now, you surely own Defense Grid: The Awakening. Even with only the most cursory of interests in tower defense games, you should have picked it up in some sale or bundle or another. If not, though, you can rectify that now with the latest pay-what-you-want Indie Royale, which also packs moody adventure game Gemini Rue and a few others. It's one of the slickest TDs around, haven't you heard?
Indie Royale's Replay Bundle Vol. 1 offers Defense Grid and a whopping six DLC packs, Gemini Rue, FPS-RTS Nuclear Dawn, and dungeon 'em ups Soulcaster and Soulcaster II plus their soundtracks. It's an encore performance for these games, hence the name, as they have appeared in earlier Indie Royale bundles.
Most games can, or even must, be activated on Steam, while some are DRM-free or on Desura.
This bundle does have an ever-increasing minimum price, but it's currently only $4.76 which is quite the bargain. Pay above $8 to get bleepy-bloopy music too. It'll be on sale until next Tuesday.