Ditto can breed with anything except pokemon on the "No Eggs" list or another Ditto.
Just in case you're wondering what those are, check here for the egg groups and anything else on breeding in R/B/Y: http://www.psypokes.com/rselgfr/breeding.php
Me, my pokemon, and I (Sapphire).
Shinies: All from Diamond.
Alchemy and Byrd, the twin Ponytas.
Hope the Shinx.