For my money, forScore is the best sheet music viewer on the iPad. It was one of the first apps of this kind on the iPad and has come a long way since its debut. It combines ease of use and simplicity with features that working musicians will appreciate, such as easy navigation, annotation features including a notes section for program notes, zoom to eliminate margins, hotspots for tricky repeats, iPod-linked audio files, and easy file transfer. Scores in PDF format can be imported via iTunes as well as through file sharing with apps such as GoodReader. If you are a pianist it comes with a decent collection of piano repertoire. The developers have been quick to enhance the feature set and hopefully they will come up with a hands-free page turning option in the future. If your looking to perform with your iPad, forScore is an essential app for your musician's toolbox.
