Tower Defence

orcs must die franchise

did you mean?: defense

Dungeon Defenders,Sanctum

Sanctum 2 and if you do get it, add me on steam :D

Defender's Quest: Valley of the Forgotten

fieldrunners 2

Defense Grid is all you need!
(Maybe give Anomaly a go for something alternative)

prime world defenders

the bestest ever made is indeed Defense Grid, every single DLC is worth to get too!
but you wisely already have it (get DLCs if not done yet)

Babel Rising
Anomaly Warzone Earth
Defender's Quest: Valley of the Forgotten
Defenders of Ardania (you should also get this one with DLCs)
Dungeon Defenders
Sanctum 1 and 2
Oil Rush (+tower defense DLC)
Orcs must die 1 and 2
Unstoppable Gorg

ibomber Defense Pacific I found pretty good. There's also Super Sanctum TD which isn't bad.

TB seemed to enjoy Go Home Dinosaurs here. Though I can't speak for it myself, as I've never played it.

Kingdom Rush.

Super Sanctum TD and Defender's Quest, for sure.

I enjoy

Defense Grid with all its DLC
Orcs Must Die 1 and 2 and all their DLC
Anomaly Warzone Earth (good reverse tower defense game)

I've got a few others but haven't gotten around to playing them yet.

Harvest: Massive Encounter!

Most of the good ones have already been said, but Tower Wars is an interesting one. You have to keep up your Tower defense while Sending out waves of your own. Quite fun once you get the hang of it. (but there is no single player though)

your steam only requirement makes me sad ;)
I`d suggest checking out McDROID but its not yet on Steam, only Greenlight

I'd suggest you check Defender's Quest: Valley of the Forgotten, if you don't mind the old-style graphics it's very addictive (and you can customize your "towers" a lot!)

there was also this game i forgot the name but it was something about titans and you had it for linux

Defense*

Anomaly Warzone Earth is crap. And I don't even count Sanctum, Dungeon Defenders and Orcs Must Die as true tower defense games. All games I know have already been mentioned: Defenders of Ardania, Go Home Dinosaurs, Power of Defense, Unstoppable Gorg, Defender's Quest: Valley of the Forgotten, Fieldrunners 1&2, iBomber Defense, iBomber Defense Pacific, Super Sanctum TD, Prime World Defenders, Revenge of the Titans, Tower Wars

I recommend Sanctum and Sanctum 2,really good games in my opinion,and Orcs Must Die is ok if you want do try it.

i'd say Anomaly 2, it's reversed tower defense in campaign, but in multiplayer you play for both sides.

oh, and get Defense Grid's DLCs. "You Monster" is a delight.

Dungeon Defenders, Orcs Must Die, Sanctum.