ChordAid (was AltiChord) is a chord reference application for various stringed instruments like guitar, bass, ukulele, mandolin, or bouzouki. It supports standard and custom tunings, simple and jazz chords. In addition to the usual chord shapes, it lets you choose among more difficult chord shapes with two barres and big stretching between fingers. The trial version is limited to few chord types in each category and limited number of instruments and tunings. The full version is available on the Mac App Store.