A couple years ago Valve hired the folks who made the Alien Swarm mod to work on their various other projects. During that time they slapped together a Source version of the game, with brand new Source engine enhancements to be released alongside. And it also comes with Steamworks integration. And it's free. And you can download it on the 19th.
 
That's pretty cool!
 
Store Page: (download/trailer/screens/details)   http://store.steampowered.com/app/630/
 
Details:

Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. With your friends, form a squad of four distinct IAF Marine classes. Plan your attack using an unlockable arsenal of weapons with countless loadout configurations against a wide variety of aliens. Blaze your way through an overrun, off-world colony, eradicating the alien infestation in environments ranging from the icy planet’s surface, to a subterranean lava-flooded mining facility.

- Tactical, 4 player co-op action game with a top-down perspective
- Complete game code and mod tools
- Unlock persistent items by gaining levels
- Over 40 weapons and equipment with countless loadout configurations
- 4 unique classes and 8 unique characters
- Matchmaking, Steam Cloud, Steam Stats
- 64 achievements
- Tile-based map generation tool
- Powered by Source and Steam

I love me some freebies.

Nice!  I like free games.     :)

It also comes with the SDK to encourage people to start making their own mods.
 
Because that's how Valve rolls.

Gosh have really fond memories of the original mod for UT2k4, Alien Swarm was a better Alien Breed game than the new Alien Breed game ever was. I wonder how good it is on the source engine, wil be fun to find out.

wow. i want it. AND i dont have to pay for it! Thanks valve  
 

I wonder if in a few years Valve will make some progress catching up to Epic in terms of engine licenscing.
 
Probably not, but at least they're trying.

That is awesome, never played the original so it will be nice to try it out.

This is the best news I've heard all day.

They want to see how far they can push this engine.  I'm all up for collecting the spoils of rigorous testing.

I also have to wonder if id Tech 5 is going to be a comeback for Carmack now that he's not putting so much effort into solving problems that apparently game makers didn't need solved.

I still love you, Valve. <3 
 
Now if you'd only release Episode 3 before the world ends, and I'd be even happier.

Awesome!

what a nice suprise!
 
<3 valve

Great, and this might be a stupid question but will it have Steam achievements?
 
I'm new to steam so I haven't figured everything out

What's it about?

Cool, not sure if it's my thing though...

" Great, and this might be a stupid question but will it have Steam achievements?  I'm new to steam so I haven't figured everything out "

" What's it about? "

Looks fun, thx Valve.

I think this is an awesome way to both encourage more developers to work with Steamworks and provide gamers with a free game. Once again, bravo Valve, I'll definitely be downloading this.

Trailer:  http://www.alienswarm.com/

Details:

Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. With your friends, form a squad of four distinct IAF Marine classes. Plan your attack using an unlockable arsenal of weapons with countless loadout configurations against a wide variety of aliens. Blaze your way through an overrun, off-world colony, eradicating the alien infestation in environments ranging from the icy planet’s surface, to a subterranean lava-flooded mining facility.

- Tactical, 4 player co-op action game with a top-down perspective
- Complete game code and mod tools
- Unlock persistent items by gaining levels
- Over 40 weapons and equipment with countless loadout configurations
- 4 unique classes and 8 unique characters
- Matchmaking, Steam Cloud, Steam Stats
- 64 achievements
- Tile-based map generation tool
- Powered by Source and Steam

this looks sweet. i'm excited. 

For those wondering aparently the 4 unique classes are:
 
Officer
Tech
Medic
Special Weapons Expert
 
Can't wait to jump in, was looking for a 4player co-op game and now one's landed on me free of charge. Imagine their business model is that a free co-op game will have people getting their friends to download Steam, more people on Steam, more sales.

Cool! Free Valve game!

Cool, might have to install Steam now.

Store page:  http://store.steampowered.com/app/630/

It looks fun, I'm recapping on some Battlefield 1943 ATM.

Hell yeah free! The game looks pretty fun too. I really hope they have some local co-op in this, but if not who cares? IT'S FREEEEEEEEE!

Gameplay:  http://store.steampowered.com/video/630

They should upload the trailer to the site so more people notices.
 
Also, if anyone wants to write something about the game in the wiki that whould be great.

The slow-mo effect in the gameplay vid is crazy awesome.

Cant wait for this, could see myself shooting a ton of aliens with friends.

MAAAAARIIIIIIIIIINEEEES WE ARE LEAVING

I've been contemplating making a giantbomb account since i started coming here in December. I played the original alien swarm mod and periodically checked in with the blackcatgames site to check for updates only to find nothing... Never could I have imagined that opening a thread talking about valve's generosity would not only contain alien swarm information but also mention the fact that it's free. 
 
FINALLY... I can hack the planet again...I'm so happy right now ;_;

Wow, I remember playing the UT2004 mod a few years back, it was a very cool little co-op game. Looking forward to playing this!

Wicked. I just loooove that price tag.

Free, everything free is good. I like free stuff..... FREEEE :)

Oh my god that looks awesome!  I like Killing Floor which is a coop zombie killing game with unlocks, so this will probably grab me the same way.

So because I have not a clue about pc gaming, will I be able to run this on my ho hum Dell PC?

" For those wondering aparently the 4 unique classes are: Officer Tech Medic Special Weapons Expert  Can't wait to jump in, was looking for a 4player co-op game and now one's landed on me free of charge. Imagine their business model is that a free co-op game will have people getting their friends to download Steam, more people on Steam, more sales. "

This is coming out tomorrow!
 
GET READY

" This is coming out tomorrow!  GET READY "

" I still love you, Valve. <3  Now if you'd only release Episode 3, the world can end. "

im so exicted! T-13 hours!

Cool, I'm always down for free games.

But can my computer run it? I think not

Cool! I will play this for sure.