CONECA (pronounced: CŌ´NECA) is a national numismatic organization devoted to the education of error and variety coin collectors. CONECA is and acronym which stands for The Combined Organizations of Numismatic Error Collectors of America.
CONECA focuses on many error and variety specialties, including doubled dies, re-punched mint marks, multiple errors, clips, double strikes, off-metals and off-centers -- just to name a few.
As an organization they have standardized a naming convention for DDOs, DDRs, RPMs, OMMs, etc. and created a master list of all these errors.
You can visit their website here. While not inclusive of every possible variety, the CONECA master listing has most of the discovered varieties for the Washington Quarter. To the left in the menu are links to major groups and their master listings.