http://store.steampowered.com/app/720270/Defense_Task_Force/

If you guys are looking for new TD games then there is Unleash by DeSync Studios. It's on Steam Early Access at the moment.

It's a Tower Wars game and it has 4 main modes: Line tower wars mode PvP and PvE, tower defense mode, and Sandbox mode. In line tower wars you can play with up to 7 other players or bots.

Here is the steam page if you wanna take a look (just add https:// in the search bar before the link)

store.steampowered.com/app/621940/Unleash/

And here is the discord server: discord.gg/unleash

Sorry to bump this, but I just bought Defense Grid on Steam at the Black Friday Sale. The wikipedia says there was a thing called "Community Levels" which was available if you put your email address. I did so in-game but didn't receive anything. The text next to it says that the "Contest" (?) db is down. Is there any way to still get a key for this?

And I got a reply from their support that that system is down but they "will try to get some time to look at getting it back up and running next week. Sorry for the delay."

If you like tower defense check the Steam sale, you can get Defense Grid with 4 DLCs (plus the Community Levels if the above is true) plus Special Edition of Defense Grid 2 (includes making of movie, artworks and other stuff) all for $9.98.

How about... spreading your spam/virus/malware somewhere else?

Pixeljunk Monsters (here) is a fun one.

For iOS, "The Creeps" is excellent.

X-Morph: Defense Complete Edition is a very good one and it is available on GOG since this spring.