Steam has released the Swarm Map Pack for Aliens vs Predator, It will run you $7.99.
The four maps included in Swarm came with the special edition console versions but the PC version wasn't shown as much love, apparently.
Two of the maps, Docks and Outpost, are for the Deathmatch, Predator Hunt, Infestation, Species Deathmatch, Mixed Species Deathmatch and Domination gameplay modes.
The other two maps, Machine and Hive, are specifically for the game's four player co-op Survivor mode.
Head on over and get to getting it.