There's a new project called Nouveau to make some open-source nVidia drivers for DRI. This sounds great as DRI seems to fit in well where nVidia just made up new architecture everywhere. From the Nouveau web-page it sounds like it's not too hard to write drivers - all you have to do is convert OpenGL calls to appropriate format for the hardware. All the OpenGL state machine work is done for you by Mesa (and is shared between all the DRI drivers so bugs get fixed once only).
Back in the day I used the Utah-GLX nVidia drivers which were quite buggy but then it was a bonus to have 3D support. The commercial ones were just that much more reliable that I eventually changed to them. I haven't bought a nVidia card since though. If these new ones turn out OK that would change :)