PSOUG.org has just launched their Oracle Error Reference tool, listing every Oracle "ORA error" in the known universe. Well, probably not every one, since Oracle seems to keep minting new ones almost daily, but this is a pretty comprehensive list of over 18,000 Oracle error codes, their meanings, causes, and remedies. The searchable list also includes error codes for SQL*Loader, RMAN, NNL, TNS, DRG, IMG, PCC, LPX, LSX, and many others.
Check it out: ORA Error Reference Library.
For example, who knew that ORA-36273 meant that a "dimension workspace object was specified more than once in the QDR"?