A month ago I bought this ear training course on CDs.
Basically it helps you to develop a perfect pitch.
If do not know what the perfect pitch is go to
http://www.eartraining.com/. Pecfect pitch - to identify
notes by hearing them. If your budget is low do not worry
getting that course. I am just saying it helps your guitar and playing your instrument in tune. Also it will improve your music reading! In that course the instructor says anyone can develop a perfect pitch. If know anyone who
has a perfect pitch ask them if they can help you.
Here is a hint if you are interested: for instance,
play any two white notes on a piano or keyboard. Then
try to sing the low note first then the highest note.
play notes within arrenge that you can sing. Start
playing notes with "short intervals" first. For example,
C and E or A and C. one more thing, your ears should be
relaxed; otherwise, it won't be effective.
Well go online and research, develop your ears.