Other
Functional Ear Trainer
This is a freeware software product that can help you develop your ear training. There is a basic and an advanced version of the program.
OpenStax
Open Stax: Catherine Schmidt Jones: Ear Training
A detailed site explaining about ear training for musicians. Included are many links to other relevant articles that go more in depth about the various aspects of developing ear training. There is also a downloadable ear training game to...
Other
Vocalist.org: Perfect Pitch
This brief essay discusses the writer's quest for perfect pitch even though he was not born with it.