I'd like to add the Raspberry Pi to the list.
http://www.farnell.com/raspberrypi/
I recently bought a Raspberry Pi. The first think I did was install MPD onto it, and it was playing music within minutes. I did cost 32 British pounds = 40.65 Euros, so about the same as and entry level router.
Plus sides:
+ It already has audio out, so no need for an usb-sound adapter.
+ 700 MHz CPU
+ 256 MB SDRAM
+ 2 USB ports
+ Can play 1080p via HDMI.
+ Composite out for old tv's
+ SD card slot, so easily extendable memory. (No more 4MB limit, but more like a 32GB limit
)
+ 17 GPIO connections, so you might be able to ditch the micro controller completely
+ Support for multiple OS distributions.
Downsides:
- It doesn't have wifi, so you'll need a usb wifi adapter.
- Doesn't include a power adapter (5v, 700mA, micro usb)
- No internal Flash memory, so requires a SD card, that isn't included
- Doesn't have an enclosure
PS Also check the upcoming APC from VIA (
http://apc.io/). A bit more expensive, and currently only supports Android.