This site lets you practice identifying scales, chords, intervals, and chord inversions. There is also an app on the App Store that I'm pretty sure is free.
For those of you that don't own a piano, this app is a portable piano for your iPad/iPod/iPhone. This is really helpful for visually seeing where notes such as C4 and the other notes on the piano fit. It is also useful for practicing ear training (like we do in class).
Sight Singing is supposedly one of the most difficult challenges on the AP Music Theory test and stunts many students. However, this site promotes voice, ear, pitch, theory, and rhythm training. With enough practice, this site will prove to be very useful. :)
I don't care if this has nothing to do with interval ear training, I just found what this guy had to say about perfect pitch really interesting. I still have no idea if my pitch is actually perfect or not though haha.
List of famous songs to learn musical intervals. Create your own custom lists with the interval song chart maker. A perfect starting point for ear training.
Sight Singing can be pretty difficult for people who aren't used to reading notes on the dot like a vocalist. At this site, you can either subscribe or practice for free (and improve such skills!).
"Cantorion is a free sheet music repository and a free concert listing diary that anyone can contribute to." This website is a good resource for written music and may assist in written theory and recognizing intervals. Also a great source for free piano music!
This website has practice exercises for everything from note dictation and clef reading to harmonic progressions and the construction and identification of seventh chords. The "exercise" page, which this link goes directly to, houses all of the practice resources.
The site interacts with apps for mobile devices/tablets to create customized flashcards. You can utilize both text and pictures, depending on the content needed.
This website helps recognize different scales by ear, without looking at what key it's in. This also helps greatly with hearing different trichords, harmonics, and melodic scales.