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.
Music Theory
Music theory.net: Interval Trainer
This online quiz is a fantastic tool to check your knowledge of intervals. Keeps score, and can be customized to fit specific lessons (augmented intervals, diminished intervals, double accidentals.)