I've added full head tracking and weapon tracking to Half-Life 2 in preparation for the Oculus Rift dev kit launch. Once the official rift apis are available I'll integrate them, giving people ~70 hours of content (this also works for episode 1 & 2, the lost coast and tons of user generated maps, etc) that they can use to test out their Rift.
This video shows a handful of scenes of me using the head and weapon tracking using two hillcrest trackers (the same ones that John Carmack used for the Doom 3 BFG demos of the early oculus rift prototype). This video was made using my modified HMZ-T1 but others have played it on DIY rifts with the vireio stereo/warp drivers. The videos aren't entirely in sync, I tried to get them as close as possible but I'm definitely no professional video editor, but the actual tracking is very responsive and accurate.
I'm currently testing different controllers, in the video I'm using a "top shot elite" that is effectively a wireless xbox 360 controller, and used electrical tape to hold the hillcrest tracker on the top.