The Rift Arcade - There's a surprisingly small number of older games that have been updated to work on the Oculus Rift.
Half Life 2, Team Fortress 2, Quake, Doom and Duke Nukem 3D immediately spring to mind, but there are few others, and when do do stumble across one there's no guarantee that it'll work properly.
That's where third-party software and drivers come in. Vireio Perception and vorpX both attempt to force games to render in stereoscopic 3D and with full head-tracking on the Oculus Rift. In the case of vorpX, they also charge a fee for the service. Today, Vireio Perception released version 2.03 of their software alongside a dedicated page at the Mean to be Seen website.