Circle Theory  

($4.99)

Designed for both iPhone & iPad

Link: http://itunes.apple.com/us/app/circle-theory/id289187333    

Circle Theory is a graphical Circle of Fifths identifying relationships between notes, key signatures, intervals, and transposition between keys.  An iPad VGA adapter can be used to view the circles on a screen or with a projector.  

Quinterval 

FREE

Designed for both iPhone & iPad 

Link:  http://itunes.apple.com/de/app/quinterval/id373810100 

Quinterval is an illustrated Circle of Fifths showing major and minor keys and intervals.  Keys can be selected by tapping the screen or using the graphical knob. Available in German, English, French and Spanish.