This page lists some of the available devices with a Mali GPU, together with some useful info about them. The GPL VIOLATOR status for most of the devices is pretty much a given at this point, so let's just mark devices as such unless proven otherwise.
Be careful where you buy, most cheap shops will not ship from your country but will ship from China. This means that you might end up paying customs, and end up wasting some time at the customs office.
The allwinner A10 and A13 SoCs are currently the easiest and best supported targets for developing an open source driver for the ARM Mali.
These devices are a Cortex A8 capable of clocking little over 1GHz, comes with lots of expansion possibilities, even SATA. It features a Mali-400MP1, so it is not a stellar performer, but it more than makes up for that in availability and price, and openness. Allwinner itself is not directly supporting open source software, and would be a GPL violator in itself. But luckily, their lack of control on their device makers made the necessary code fall out through the cracks, and they are the most compliant of any chinese SoC maker today.
There is a very active open source community, called linux-sunxi, to support these SoCs, and device support is growing rapidly. Check out the main linux-sunxi page to find out about the supported, and sometimes even fully open source, hardware available.
Cubieboard (Open Source Hardware!)
The Cubieboard comes with 512 or 1024 MB of DDR3 RAM, 4 GB of NAND flash storage, a microSD card slot, Fast Ethernet, USB host ports, a SATA port, HDMI output and can be had for as low as 49 USD.
The Gooseberry board is actually a tablet board. It comes with 4 GB of on-board storage, 802.11n Wi-Fi, HDMI, and a microSD card slot. Android 4.0 "Ice Cream Sandwich" is officially supported.
The Hackberry development board comes with 1 GB of DDR3 RAM, 4 GB of NAND flash storage, a full-size SDHC card slot, Fast Ethernet, USB host ports, built-in 802.11n Wi-Fi, HDMI output and can be had for 65 USD.
AMLogic 8726-M (Mali 400)
Zenithink ZT-280 (GPL VIOLATOR)
The ZT-280 range includes the C71, a 7" tablet with a capacitive display. Can be had for under EUR 100 these days, but add customs and postage to that.
Point of View ProTab 2XXL (GPL VIOLATOR)
According to the spec sheet provided by its manufacturer/reseller, the ProTab 2XXL features a Mali-400 GPU. This tablet features a 10" capacitive touch-screen, and is very competetively priced - it retails for about EUR 170. Point of View publishes "Firmware Updates" in its somewhat chaotic download area, but there's no source code in sight anywhere.
Exynos 4 (GPL VIOLATOR)
These SoCs are the best performing Mali-400 devices out there. They are proper speed-daemons. The exynos 42xx series has a dual A9, whereas the exynos 44xx series has a quad A9. All come with a Mali-400MP4.
All exynos 4 devices come with binary only u-boot. This means that Samsung, and its device makers, are violating the GPL.
Origen Board (GPL VIOLATOR)
ODROIDs (GPL VIOLATOR)
The Odroids are small developer boards with many possible connections. The Odroid-x2/u2 is hyperfast, as it can clock the 4 A9s to 2GHz, and the Mali-400MP4 can clock up to 640MHz. This makes for a nice high-end benchmarker, and a good comparison for the comparatively meek A10.
Hardkernel tries to portray itself as open source friendly, but they have a lot to learn still. They are providing some sort of crazy android and ubuntu pre-made SD-card images, and even hand out Mali binaries for ubuntu. Hardkernel knows the pain of getting the Mali binaries built and integrated, yet they are not interested in cooperating with our project. They officially claim to have "community based" support, and we all know what that means. Since these devices are developer boards, they have a much longer life span than your average mobile phone or tablet. In the mid to long term, hardkernel, well, hardkernels customers, will end up depending on the support of the lima driver project.
Here is some information on how to set up your own SD card with a custom built kernel and with mali binaries (which we need for reverse engineering).
Samsung Galaxy S II (GPL VIOLATOR)
Samsung Galaxy S III (GPL VIOLATOR)
This SoC incorporates the Mali-T604 GPU along with 2 Cortex-A15 cores.
Samsung Chromebook XE303C12
This is, as of December 2012, the only ARM-based Chromebook. It costs 249 USD.