Rebellion's boss promised the Steam re-release of 1999's original Aliens vs Predator would support multiplayer. They've delivered, patching in the support for that today.
Here's what it will and won't do:
• Integrated with Steam Overlay
• Uses Steam name
• Host/Join games via bare-bones in-game lobby browser
• Can invite and join friends and others via the Steam overlay
• Game attempts host migration
• Dedicated servers are not supported or planned
The game is $US4.99 but if you already bought it, the multiplayer patch will be served for free.
Aliens vs. Predator Classic 2000 [Steam]