How about a Chromebox? Don't let the name fool you: they're completely open, so you can replace ChromeOS with Linux, or even dual-boot between the two.
$159 gets you a dual-core 1.4GHz Celeron, 2GB of RAM, a 16GB SSD, four USB 3.0 ports, an SD card slot, HDMI, DisplayPort, Bluetooth, 802.11n...