All of these are free. If you have better versions or more ideas please let me know! I dont include any apps that cost $$.
In no particular order:
Classical Guitar: Free part of the Six Strings app. Easy to use, great sound. Best of the many free guitar apps we have seen by far.
Mandala HangDrum: Cool steel drum type of instrument. Can create many different octaves. Cool for more visual type of kids. We use this for pentatonic scale work. I ask them to invent something here then play it on piano.
PraiseHymns: Super, super cool (as a choir director!). Hundreds of hymns loaded in 4 part harmony. See sheet music right on ipad and it plays the tune! Great for sightreading, analysis, and tune research. Must have.
GrooveMaker: Simple loop machine. Comes with one package and they want you to buy more loops. It works really easy and anyone can create some nice loops. Kids love playing with this.
Beatwave: Simple visualizer. So cool. Create loops using your finger and pressing dots. I have students create loops on Twinkle Twinkle. Love this.
Aweditorium: Awesome "browser" for music. Has thousands of pictures on screen. Click a picture and goes to band. Great way to explore new music.
Ear Trainer Lite: Theory ear trainer. Does intervals in game/quiz format. Simple and good.
JamPad: Simple piano. Can create drum backgrounds and chordal backgrounds. Kids use this alot.
NLog Synthesizer Free: Simple synthesizer. Students use to create horn sounds they cant find on other apps.
BarrelTones: Best drum app we have found. Great response. AND you cna play along with recordings.
SoundHound: Jaw dropping if you havent seen this yet. Play any song and it will tell you its name. No more drop the needle tests. Amazing. I love having my fourth graders sing into it and having the song come up.
Radio.com: Thousands of real radio stations. Want to know whats playing in Istanbul? Or miss your college radio station? Great tool for music classroom.
NPR Music: The coolest and hippest. Greatly improved interface. They spend a lot of money recording and searching for the great music.
LoopJ: Loop dance machine. Use fingers to move between loops and beats. Surpisingly effective.
Progression: I dont understand tabs, but this is very intuitive and easy to use notation software.