Other
Big Ears: The Original Online Ear Trainer
This is an excellent resource to help you identify intervals. You can choose which intervals you would like to hear and it will play them for you. You can also play intervals on the virtual piano and it will tell you what they are.
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: Interval
Learn about simple intervals and compound intervals in this well writtern lesson. You will also learn how an interval is classified. Included are several self checking problems.