A while back I posted a quick review of Truefire's 50 Blues Guitar Licks app for the iPad. If you haven't picked it up, you really should. For $5.00 that's 10 cents a lick. Even if you just learn 10 things from this application it's a steal.
TrueFire's not stupid. They know that if you test out this application and enjoy it, you'll come back and buy other applications also. They've also released several more "50 Licks" applications of the iPad including Blues Rock, Rock Guitar, Jazz, Shred, Country, Metal, Funk. I've picked up several of these, and they are all terrific. These aren't just crappy ports of their Mac/Window versions either. They are nicely set up on the iPad and make the best of the iPad scree to show the video and tab on screen at the same time.
And if you like the iPad versions, you might also be interested in their Mac and Windows applications. I've picked up several and they're great.