I use Complete Ear Training for interval training, with the grand piano or the Rhodes sound pack. I switch pack every once in a while. I also use TonalEnergy Tuner to sing intervals or scales to it and see if I'm in tune.

In something like a year I went from "telling apart major and minor thirds is black magic" to being able to sing the major scale right like 60% of the time and I have an 80% success rate for all intervals up to a fifth. It did improve my piano improvisational skills by a lot.