"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 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. :)
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.
Learn the names of the lines and spaces of the bass clef with this interactive online game for free.
Saw one for treble and figured we could use a bass clef as well!
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 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.
At around 50 seconds in, the trumpets go from an F to Bb, back down to an F. This is a Perfect 4th. Subsequently, they split where firsts go to a C, while, the seconds go to an F again, creating a perfect 5th.