The Pie has FreeBSD and other Linux distro support and lots of i/O to hook up other peripherals.
And I was running Ubuntu 14.04 LTS on a Beagle Bone Black in April of '04 (although its userland was running on a somewhat back-versioned kernel for a couple months until the guy doing the kernel ports got the proper one fully ported).
The Black is not the first Beagle Bone version, either, and it was running Debian Linux from the first time I encountered it. It has lots of I/O hookup opportunities - including onboard USB, Ethernet, video, and lots of GPIOs that can be configured to provide several serial ports and a number of buses, in addition to lots of wiggle wires. And you can stack peripheral boards on it, as well.
Plug in a wall wart, USB hub, keyboard, mouse, monitor, (and, if 4 or 8 Gigabytes of file systems feels too cramped, a USB drive or mount a filesystem from a fileserver). Bingo: a full-blown desktop system with about the power of a cellphone and smaller than a pack of cigarettes (excluding all the stuff you plugged into it, of course).
Which is not to say it's the best choice. it's just one I happen to be familiar with. There are a number of single-board machines out there. Cellphone processor technology is too powerful, cheap, and available to NOT be plowshared.