The Audioengine D1 recommendation from Chayro is a good one. It has USB and optical inputs, the optical input made it attractive to me for connecting to an Airport Express for streaming to a small workshop system.
If you need a coax connection rather than optical the Nuforce Icon UDAC or UDAC2 have that plus USB in a small, inexpensive package.
If you need a coax connection rather than optical the Nuforce Icon UDAC or UDAC2 have that plus USB in a small, inexpensive package.