Looks like http://swarmarmory.com/ is the place to go for maps and ♥♥♥♥.

There's also a small map pack with some of the maps at http://forums.steampowered.com/forums/showthread.php?t=1374591 but they're probably all from there anyways.

On a side note, some of these maps really ♥♥♥♥♥♥♥ suck.

Alien Swarm Update Released
Product Update - Valve
Updates to Alien Swarm have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

Alien Swarm
Matchmaking distinguishes between games in progress and games in the briefing
Updated Hammer’s simple compile options
Fixed fast reload sounds and incidental marine conversations

Product Update - Valve
Updates to Alien Swarm have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

Alien Swarm
Fixed crash to desktop on certain graphics cards
Fixed matchmaking sending you to the wrong mission
Fixed not being able to earn the “Ammo Technician” achievement
Fixed not being able to open particle systems in the editor

Product Update - Valve
11:58
Updates to Alien Swarm have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

Alien Swarm

* Fixed weapon switch key in the 360 controller config
* XP and level are stored in the Steam Cloud
* Fixed duplicate rows in the "Find Public Games" screen
* Reduced screen shake when you have high ping
* Fixed dedicated server always starting in LAN mode
* Fixed custom item button binds not displaying properly on HUD
* Fixed full network updates causing duplicate client ragdolls to spawn
* Added console commands to change game access on the fly (make_game_public and make_game_friends_only)
* Fixed welding icon sometimes getting stuck above player's head
* Fixed HUD elements being cut off in ultra wide resolutions
* Removed cheat flag from a number of cosmetic console commands
* Now firing, meleeing, or reloading while welding will break you out of the weld
* Fixed spectators earning achievements
* Fixed infinite ammo exploit
* Fixed XP exploit

See you in-game bros!


About the Game

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.

Along with the game get the complete code base for Alien Swarm that features updates to the Source engine as well as the SDK. Alien Swarm adds 3rd person camera, depth of field, improved dynamic shadows and a wide variety of gameplay additions to the Source engine.

* 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