From the developer who brought us RunloopSound, Csound for iOS, comes Plectrum, a classic emulation of plucked string instruments. It is impossible not to fall in love with its deep and complex sonority. Plectrum supports USB and Bluetooth MIDI devices and works both as a standalone application, and as an Audio Unit plugin for GarageBand and many other hosts.
Plectrum is free on the app store now:
PalmSounds 