Apps I really like on the iPad:
1) iReal b -- used to be called iRealBook but they had to change
the name. It also used to come bundled with tons of music.
However, you can still download that music from their website.
This is a great, great app for learning and having fun as it
creates backing tracks on the fly.
2) forScore -- really nice app for organizing your tunes that
are in PDF format. Often I convert my scores and chord sheets
to PDF just so I can drop them into this program.
3) Ultimate Guitar Tabs -- great for pulling up chord sheets
anywhere (anywhere you have an Internet connection, that is).
If you do any jamming and need instant access to some music
you haven't played before, this is what you need. Requires a
modest subscription fee.