Sanctum is one you must check out. There's also Defense Grid: The Awakening.

Orcs Must Die, Dungeon Defenders, Tower Wars Beta, Defense Grid, Sol Survivor, Sanctum.

kingdom rush ... search it on google :D

There is also Anomaly Warzone Earth, tho it's reverse tower defense.

And +1 on Defense Grid, probably best pure TD on Steam.

Sanctum, DDs, OMD are more of action games with TD elements. But they're quite nice anyway! And if those are mentioned then ... I guess, Dota2? :)

Agree about Defense Grid, really cool game.

Plants vs Zombies

I have to recommend Dungeon Defenders it's one of my favourite tower defence games. That game is so awesome on so many levels. Solid gameplay, varying difficulty levels, a wide array of levels and different challenges, level-up system that offers many possible character builds and to top it off, cute characters and appealing cel-shaded graphics (at least to me :D).

The game's main story is not too long, but there are tons of collectible equipment and pets, many challenges to conquer, and of course almost a hundred Steam achievments to achieve. And if you happen to run out of playable content, there are lots of DLCs to buy.

You can also gather up friends and play in local or distant (or what is the opposite of local??) multiplayer games. In most levels you'll be playing cooperatively, but in some levels you can even fight against your friends!! If you don't have any friends who play DunDef, you can also join any public game and play with complete strangers (and maybe even make some friends of them :D).

So, I'd suggest you try Dungeon Defenders, it's a ton of fun!!

Plants vs Zombies, Sanctum, Defense Grid.

I also recommend Plants vs Zombies and Defense Grid. Great games.

Maybe Revenge of the Titans?:D

Tower wars beta
its multiplayer like some war3 maps where you were building towers and sending units at your opponents as well

I don't play alot of TDs but if you liked OMD there is now Orcs Must Die 2
and Revenge of the Titans is nice and retro as well.

cubemen. it can get a bit challenging in the final levels.
also unstoppable gorg is a good one. really challenging too

Plants Vs Zombies is the best, and then comes Defense Grid I think

Defense Grid the Awakening. One of best buy I've spent. The devs are still alive and kicking, and currently they are preparing for new expansion of the game, and doing a kickstarter to make new sequel.

Also you might check Anomaly: Warzone Earth, where you play as humans invading towers of aliens.

Tower Wars is a pretty good multiplayer TD, currently in beta. It's every easy to get a key, just join the Tower Wars Steam group and ask fellows with the tag [SVS] for a key.

Not gonna bother stating what everyone else already mentioned, although I highly highly highly recommend Defense Grid out of all the TD games.

Also, Revenge of the Titans. I'm surprised that hasn't been mentioned, it's a charming TD.

Defense Grid (=The BEST), Sol Survivor, Sanctum. And reverse TD Anomaly: Warzone Earth.

Orcs Must Die

Toy Soldiers; Revenge of the Titans; Cubemen;

Defence Grid would be the AAA of TD. Also Sol Survivor is a good game. DG2 kickstarter is on by the way, I don't think they'll get enough for the full project though.

Orcs Must Die is a real orc-massacre fest, and the second is coming in a few days, but it's not a pure TD but a TPS TD hybrid.

Sanctum is the only tower defense game I have on steam (I think). It's great.

edit: Also have Anomaly Warzone Earth, derp. It isn't bad and it's reverse tower defense. Pretty cool music and stuff.

Defense Grid is by far the best TD game on Steam. And I've played all the others. It's not only beautiful and has good sound, it's also tatically very interesting, nicely balanced, completely bug free and soooo many game modes per map.

Defense Grid is really good and normal tower defense game. Anomaly: Warzone Earth is also nice (it' reverse tower defense). And a nice non-steam online multiplayer TD game is CreepTD,

SMNC, Defense Grid, Kingdom Rush, Leave my gems alone*

*Free

FIELDRUNNERS!!!!!!

http://store.steampowered.com/app/209690/