Xiao Yixiang has String Tuner (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE (regularly $4.99) for a limited time.

String Tuner is the 1st chromatic tuner that especially designed and optimized for string instruments as: Violin, Viola, Cello and Double bass. Reference sound/Tone generator is provided to help you do coarse tuning. Moreover, with the optimized algorithms up to ±1cent precise , it helps you to do fine tuning quickly with the Build-in or
external microphone on your iOS Devices.

Most chromatic tuner used 12TET(twelve-tone equal temperament), but you know that string instruments as violin usually tune as perfect fifths against each string. That’s why the default temperament of String Tuner is set to Pythagorean Tuning. You can still customize to 12TET or Standard Just Intonation when needed.

String Tuner also supports adjustable calibration A4, you can either adjust it manually or using auto-detecting, it is really helpful when your string instrument need to tune based on Piano’s pitch(for example). Hit the A4 key of the piano and String Tuner take care of the rest.