A while ago I have been notified about a very interesting DIY project at HiFiDuino: the Soekris R-2R DAC. The DAC is based on a set of Burr-Brown PCM1704 DAC chips (one of the best multibit chips, now out of production).
The DAC has the potential to be class leading for a very affordable price, given its open source nature.
The Soekris R-2R DAC is extensively described at HiFiDuino Website:
The article is a bit old, and in the meanwhile kits have become available on Soekris website:
There is a very extensive discussion on DIY Audio, deserving to be followed:
Most likely, fixes and improvements of the Soekris R-2R DAC will released in future revisions.