The Audio Widget project has implemented working firmware and hardware for USB DACs and ADCs. On Linux and Mac it plays audio up to 24/192, on driverless Windows up to 24/48.
The firmware runs on an AT32UC3A3256. The hardware uses precision crystal oscillators and an ESS ES9023 DAC to give high-quality playback.
Programmers can play with the code, and analog hackers are free to modify the hardware. Schematics are also open source, and the boards have been made to promote modding. If you don't program or solder, just use it to listen to your music.
Source code repository: