Sentinel III homeworld is pretty darn good, it's the best in the series and more colourful than previously, and the variety is pleasingly diverse.
Dungeon Defenders is the latest TD I'm aware of, I'm currently downloading now so haven't any opinion of it, but it is using a very lovely game engine and it's multiplayer too.
Both will eventually be universal binaries (sentinel 3 still to be updated) which is nice. If you haven't already, do try towermadness, it's unusual for sure, but you won't find a more varied, deep and downright compulsive TD (unless dungeon defenders is as good as I hope)
EDIT : forgot to mention ibomber defence, hard as nails and nicely presented, different enough from the others I've mentioned to warrant a purchase
Last edited by Smartbomb; 12-15-2010 at 10:15 PM.
Reason: One more thing...