Tower Defence

Dota 2.

Sol Survivor and Orcs Must Die! 2 are my two recommendations out of all the ones I've played on Steam.

Sanctum (Sanctum 2 is good, but I like the original better, especially as a tower defense), Unstoppable Gorg, Plants Vs Zombies, Defender's Quest, Dungeon Defenders (this one is quite popular), Revenge of the Titans are some good bets as well

Fieldrunners 2

Defender's Quest - gameplay/introduction video here and a browser-based demo here if you don't want to bother downloading it on steam.

Orcs Must Die,
Orcs Must Die 2 - both have demos, try them out. Also, a Let's Play video here of the first game.

Get the franchise pack for Orcs Must Die, as both games are worth the money and having them both will provide you with additional content in the second.

I've gotten over 50+ hours from Defender's Quest alone, and over 20 from the first Orcs Must Die game(still haven't completed the highest difficulty). I recently got the second due to the sale and quickly invested 6 hours into it, playing well into the early morning before I realized just how much time had passed.

Bonus: Defender's Quest and OMD 2 have mod support through steam workshop.

Plants vs. Zombies

Revenge of the Titans is a lot of fun

Fieldrunners and Fieldrunners 2!

I have Fieldrunners for the PSP and I've loved every minute of it so I had to get it on Steam as well.

Defense Grid
Anomaly 2 (it's practically the other way around, you're the attacker, not the defender! :D)
Orcs Must Die 2
iBomber Defense (and iBomber Defense Pacific)

Listed the way I like them from top to bottom! :D

Anomaly Warzone Earth + Orcs Must Die! - Single Player Only
Anomaly 2 + Orcs Must Die 2 - Co-Op is awesome

Tower Wars (just online)
Dungeon Defenders (offline and online)

Sol Survivor, Defense Grid, iBomber Defense

Dungeon Defenders and Sanctum!

Revenge of the titans :)

Dungeon Defenders is the number one best Tower Defense game on Steam.
I have 3098 hours of gameplay on it.

Anomaly: Warzone Earth free steam game key.

i got mail from bundle stars.

'' It's been a little while since our last epic bundle, so to keep you busy this weekend we are giving you the amazing Anomaly: Warzone Earth from 11 Bit Studios!

To get your free Steam key for PC, Mac or Linux, all you need is a Facebook account. ''

http://woobox.com/vsxpyw

i liked their page and got game

Try Sang Froid : Tales of Werwolves it has a great atmosphere.

DOTA 2! :D

Orcs must die and orcs Must Die 2, not exactly Tower defense but it has a similar feeling.