Cyrus or Rega amps in their shoebox size would be a good option. I use a Cyrus 6 DAC with my desktop PC, and it fits on my desk nicely. Some Cyrus amps which have a DAC have a usb input, and so you don't need a decent sound card in your PC. Simply hook up via usb and the Cyrus DAC will do the rest.
Cyrus amps can be found with some good deals on ebay too, as well as Rega.
Audiolab M-One is also a nice little amp.
Some of these options can be quite expensive though if buying new.