There are so many interesting options for hackable ARM devices, I don’t really know where to start, so I’ll start with the ODROID-X.
The ODROID-X is a development board based on the Samsung Exynos4412 quad core ARM Cortex-A9 SoC with a Mali-400 quad-core GPU. The board adds 1GB of RAM along with 6 USB 2 ports, a 10/100 ethernet port, micro HDMI and analog audio audio in and out. It sells for $129.
This makes it one of the faster options available, but also one of the more expensive when compared to many of the Chinese options. It also has the advantage of already supporting a full Linux distro (ubuntu), whereas many of the cheaper options currently only have support for Android.