ThumbJam

ThumbJam is a powerful musical performance experience. You get over 40 high-quality real instruments multi-sampled exclusively for this app. Hundreds of included scales allow you to effortlessly play in any style from rock to classical, whether you are a musical novice or a seasoned professional.
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Jordan Rudess, the world-renowned keyboardist for Dream Theater says:
"ThumbJam stands at the very top of the overcrowded music app world. The sounds included absolutely rival those found in pro instruments and making music on the app is a total joy. The only problem I have with this app is that I did not make it myself! Luckily, I'm able to work with the developer and offer a few good feature ideas as well as some choice sounds from my personal collection!"
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Even if you've only played air guitar, ThumbJam lets you sound like a pro. Pick an instrument and jam -- with all the scales to choose from you'll never hit a wrong note. The high quality instrument samples will make you sound like you just skipped years of music lessons.
For the performing musician, this app is awesome for soloing on an instrument you didn't bring along. Play in any key, any style -- go from a string ensemble to a wailing guitar. ThumbJam was developed and tested by musicians to feel and sound like a real instrument. You'll be surprised at how expressive it can be, make sure to check out the videos and audio at thumbjam.com.
ThumbJam makes use of tilt and shake to add vibrato, tremolo, note bends, and volume swells for more realistic and expressive results. It supports up to five (eleven on iPad) simultaneous touches and up to 32 voice polyphony, for each of up to 8 loaded instruments! Delay and stereo reverb add depth to the sound, and a resonant lowpass filter gives you more tone shaping power. Put on your headphones or plug in to your home stereo to experience some of the highest quality instruments in the App Store, even after all these years!
Build your own loops from the ground up or import your favorites and jam from there. Layer as many loops as you want, with different instruments for each. Export your loops as audio files via WiFi. Copy and paste audio from other apps using AudioCopy/Paste as well the general pasteboard, via email, AudioShare, iTunes Document Sharing, Files, or open directly in another application. Supports Audiobus and InterApp Audio for streaming sound to/from other apps in real-time, including state save/restore with Audiobus 3.
Extensive MIDI input and output support over WiFi or Bluetooth, with any CoreMidi compatible device, or other apps on the same iOS device with virtual MIDI. Now also supports Ableton Link for tight sync between apps on the same network. Supports MPE input and output (and did so before it was even a thing)!
New powerful arpeggiator feature lets you play patterns with perfect time sync, from the screen or via MIDI control.
The app includes a huge variety of scales, from western to eastern. Save your favorites for quick access, or create your own in-app. You can even import new scales in the Scala format for complete flexibility.
You can also record your own multi-sampled instruments using a built-in or attached microphone, or from file import.
Included instruments:

+ Cello, Violin, Viola
+ Upright Bass (plucked and bowed)
+ Several drumkits
+ Darabukka
+ Djembe
+ Electric Guitar
+ Electric Bass
+ Trumpet
+ Trombone
+ Tenor Sax
+ Flute
+ Low Whistle
+ Acoustic Guitar
+ Hammered Dulcimer
+ Mandolin
+ Cittern
+ Grand Piano
+ Drawbar Organ (2 types)
+ Rhodes
+ Pipe Organ
+ Synth Strings
+ Synth Choir
+ Theremin
+ Sawtooth Waveform
+ Sine Waveform
+ Triangle Waveform
And from Jordan Rudess:
+ JR Zendrix
+ JR Smooth Steel
Many more for immediate FREE download in the app including several drumkits, Mellotron, basses, banjo, tabla, ukelele, guitars and more! Go to the Sound menu under Download Samples....