There's a really neat synth for iOS called Fingerfiddle. It uses physically modeled string synthesis to create a rather realistic cello (other instruments available via IAP). It uses finger motions for bowing. It's very expressive, as modeled instruments tend to be with good controls (I could never get into using a mod wheel to simulate bowing, and this feels far more natural). I think it is the closest thing to having a stringed instrument without paying hundreds of dollars for an actual cello/violin/etc. (the app cost me $3.17, and I can play it better than my cheap-ass eBay violin).
http://www.fingerfiddleapp.com/