I studied at a conservatory and we always used song mnemonics for intervals. Star Wars for a perfect fifth, Jaws for a minor second. Couldn't find a clean web app that does this, so I built one in a day.

What it does:

Interval and chord recognition with 12 intervals and 8 chord types

Spaced repetition (asks more of what you get wrong)

4 sound types: piano, synth, guitar, sine wave