After having released 2D hardware registers informations for their R5xx and R6xx GPUs a few months ago, AMD/ATI has just published the first bits of open-source 3D programming documentation for their R5xx GPUs (Radeon HD X1000 class cards). Even if R5xx are pretty old now, AMD plans to released 3D specifications for their R6xx (Radeon HD X2000 class cards) last architecture (supporting SM 4.0 and featuring unified shader architecture like the G80) very soon. This release is part of a large AMD plans to open up their whole hardware allowing the development of an open source driver fully supporting the last ATI chips (more infos here). As ATI have never been able to develop correct OpenGL support (nor linux drivers), it sounds for me also like a very good news for OpenGL support on ATI hardware. Who know maybe will we get OpenGL 3.0 support on ATI before on NVidia hardware... Hum, sounds like science fiction !

As for Intel's documentations published a few weeks ago (see here ), these documentations gives very interesting details on the hardware architecture but this time for a "true" high performance 3D architecture. Among the areas covered in this guide are the general command processor, vertex shaders and fragment shaders, Hyper-Z, and the various 3D registers.
In addition to these specifications, AMD should release soon the source of their new proprietary OpenGL implementation as well as their "TCore" tool, a kind of GPU emulator used internally to test software code before chips availability.

The R5xx documentations can be found here: http://www.x.org/docs/AMD/

More informations here.