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. :)
"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 lists intervals and shows them on the treble clef. There are also tabs on the side to notation, rhythm, meter, tempo, musical form, scales, chords, and harmony.
This is a comprehensive study guide for many concepts within music theory. Given specific parameters, it will generate specific scales, key signatures, etc. It also has manuscript paper, glossary of vocab terms, and more.
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!).
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.
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 has key techniques to help with sight reading and interpreting the piece in your mind faster. This website also helps understanding sight-reading on different levels, not just note by note.