Three of the new OpenGL extensions that were added for Mesa 7.5 are GL_ARB_framebuffer_object, GL_EXT_vertex_array_bgra, and GL_NV_texture_env_combine4. These extensions are implemented in all Mesa software drivers, but when it comes to hardware acceleration right now it's just found in the Intel i965 3D driver. The two-sided stencil support for OpenGL has also been reworked so that the drivers can implement GL_ATI_separate_stencil, GL_EXT_stencil_two_side, and OpenGL 2.0. The ATI R300 driver has implemented GL_ATI_separate_stencil.
Besides the mentioned new features, other work has already went into Mesa 7.5 too. Just in the past 48 hours have been over 60 commits to its master branch found at FreeDesktop.org.