I won't pretend to be a legitimate guitarist, because I'm most certainly not, but Perfect Guitar Song Book seems like a very useful app for those that are. It's got all of the usefulness of an entire library of tablature, but none of the mess or back-breaking physical requirements that come with lugging it around. Of course, that's what roadies are for, right?
It's not all about portability, though. Perfect Guitar Song Book can also be used to grab and format notes and lyrics from any number of websites directly through the app. The text and fonts can also be tweaked to allow for easier reading, then saved for later use. Favorites can be exported into various formats (.PDF, Chord Pro, etc...), which I can see being a big deal for potential song writers.
The app also offers the useful hands-free option of scrolling via shaking. So if someone's presumably in the middle of playing they don't have to stop in order to "turn the page" as it were. The app also supports TV-Out, so with the proper cables a user could easily put the song up on a big screen for the rest of their band-mates/friends to use as a guide.
If Perfect Guitar Song Book sounds like a great idea, then feel free to go and grab it off of the App Store. It's available right now for $3.99 for all major iOS devices.