As someone who doesn't own a soldering iron, I've found the $35 Raspberry Pi board [1] [2] [3] to be an excellent choice as a low-cost low-power general-purpose ARM-based server.

It also has a number of GPIO pins, UART, i2c and SPI, which may be of interest to those who do own soldering irons :)

[1] http://www.raspberrypi.org/faqs

[2] A model with no Ethernet is available for $25, but Ethernet is kind of non-optional for a server :)

[3] The price doesn't include power supply, case or boot media.