ARM GPU Overview

There are different GPUs used in current ARM SOCs.
For nearly all GPUs, OSS enthusiasts tried to create open software drivers.

Nivida Kepler

is used in Tegra K1and is quite well supported by Nouveau project.

Adreno

is used in Qualcomm SOCs. The Freedreno project by  Rob Clark tries
to create an open source driver.
This seems to be currently the most advanced project.
https://github.com/freedreno/freedreno/wiki

VideoCore (VC4)

is used in Raspberry PI. Broadcom has hired Eric Anholt to create an Open Source Mesa and DRM Kernel driver. Latest status can be found here
http://dri.freedesktop.org/wiki/VC4/

Vivante

is used in SOCs from Freescale, Marvell and a few other companies.
See  https://github.com/laanwj/etna_viv
for latest status.

Mali

is the GPU by ARM and used in SOCs by different companies. The
Lima Project by Luc Verhaegen tries to create a free driver for
M200/400/450 GPUs while there seems to be a still unreleased TAMIL project for newer T6xx and T7xx GPUs in the works.

PowerVR

is GPU by  Imagination Technologies used in SOCs by different companies.
http://powervr.gnu.org.ve/doku.php. 
Currently least supported GPU. But maybe this will change because

Imagination Technologies tries to compete the  ARM boards with their MIPS Creator CI20 SBC.