Edited 6/15/13: I see these posts are still getting a lot of traffic, two years later! Much of this info is outdated. However, I am working on updating my iPad app pages for 2013! I'll post to my blog as updates are completed, or you can check the page directly.
It's tough to even scratch the surface of the multitude of musical instrument apps in the App Store. I've not included some of the hardcore apps such as the amazing Korg synthesizer apps, Reactable, etc., because they are really intended for serious electronic musicians and aren't as suited to use in the lower and middle school classroom. I'm sure I'll be updating this list frequently!
- Aeolian Harp: This is a beautifully relaxing instrument that creates gorgeous images as you play. More for personal meditations than use in the classroom.
- Accordéon: This app takes over your screen and turns it into a full-sounding accordion!
- Bebot: This adorable little robot is actually a highly functional synthesizer.
- Celtic Harp: This full-screen, customizable harp has a beautiful, soothing sound.
- Glee Karaoke: If you don't mind the "Glee" branding, this is the best Karaoke app for iPad. Progress through songs and try to beat your high score (based on pitch and rhythm), and listen to top performances from around the world!
- iGuzheng: A nice introduction to a popular Chinese zither. This app features beautiful demo songs and multitouch functionality for different effects.
- iTabla Pro: If you are an Indian music performer or enthusiast, iTabla Pro is an incredible practice partner. 32 fully configurable taals are available, with table, two 5-string tanpuras, a shruti box, and more. iTabla can auto-tune to match your instrument. This is yet another example of an app that exceeds the performance of a dedicated hardware device, for a fraction of the cost. As far as apps go, though, this one is pricey ($25).
- Magic Fiddle: Smule's Magic Fiddle is an expressive little virtual bowed stringed instrument. Go through a series of lessons, and share your performances with the world.
- Magic Piano HD: An assortment of simplified keyboards, "light beams" to help you play classic tunes, and a social networking component make this another addictive app from Smule.
- Melody Bell: I grew up playing handbells, so when I saw this app as part of the North Point iBand Christmas extravaganza, I had to have it! It's not a universal app, but the iPhone app looks great and is easier to play on the big screen.
- Ratatap Drums: There are many virtual drum sets in the app store, including the nice one built in to GarageBand. Ratatap is one of my favorites of the dedicated drum apps.
- ThumbJam: ThumbJam is a unique instrument designed completely for the iPad (and iPhone). 30 high-quality sounds are included, and in addition to your fingers (and thumbs!), this app uses the internal gyroscope as a controller. It also functions as a looper, and is compatible with AudioCopy/Paste. The configuration options and the fact that it doesn't rely on knowledge of real instrument technique make it excellent for the general music classroom.
- YumiSynth: This is a unique bowed string simulator, in which one hand "bows" the instrument while the other fingers the notes. YumiSynth can be a highly expressive instrument with a little practice.
- 50 in 1 Piano HD: There are probably hundreds of piano apps in the app store. Out of the ones I've tried, 50 in 1 Piano is one of my favorites. In addition to a good piano sound, it has 49 more high-quality sampled instruments, effects, drumbeat loops, and 120 piano lessons, and more.