Complete an achievement in Alien Swarm and you'll unlock a head-mounted alien parasite in Team Fortress 2.
If you've completed at least two campaigns on any difficulty, this'll drop for you in TF2. This is the first time Valve have done that - searched another game's achievements before gifting you a hat, that is. I can only hope that all the best and worst achievements in Steam games across the board have their own hats soon. Imagine a gnome hat for those annoying gnome achievements Tom keeps intractably marching off to complete!
Thanks to x25killa for the tip and for posing for the shoot.