AMD Open-Source Fusion: The AMD Fusion "Ontario" support had open-source Linux support released in late November. The Mesa / Gallium3D driver changes for the Evergreen-derived graphics on these first-generation AMD APUs have been in the mainline code-base, but now with the 2.6.38 window open, the kernel DRM portion is landing.
AMD Open-Source Radeon HD 6000: It was just last week that AMD released open-source Radeon HD 6000 series support for their "Northern Islands" GPUs (not the Cayman series). Like the Fusion support, the kernel bits to the Radeon HD 6000 series are landing in the Linux 2.6.38 kernel. Some distribution vendors may back-port the code as well.
Nouveau Fermi Acceleration: With the Linux 2.6.38 kernel there is initial reverse-engineered 2D/3D support for the "Fermi" GeForce 400/500 series of NVIDIA graphics cards. Previous to the Linux 2.6.38 kernel there was kernel mode-setting support, but with no 2D/3D/X-Video acceleration. Though taking advantage of Fermi open-source acceleration currently requires out-of-tree firmware.
Radeon Page-Flipping: Support for the Radeon DRM to utilize KMS page-flipping support has finally landed in the mainline tree after it's been available with Intel's DRM for a few releases now and then more recently has been Nouveau page-flipping. The page-flipping support should improve the open-source driver performance.
Nouveau VRAM/VM Rework: There's a video memory rework found within the Linux 2.6.38 kernel.
Intel Power Savings: Users of Intel IGPs on Linux will benefit from greater power-savings with Iron Lake and Sandy Bridge chipsets. There's also full GTT support now for the Intel DRM.
Besides these specific improvements, the Radeon DRM also has PCI Express 2.0 support and the core DRM code now also has support for high-precision vblank time-stamps.
The pull request for the Linux 2.6.38 DRM can be found in this mailing list message. It's another good round for open-source GPU drivers and more benchmarks will be on the way soon enough.