Guitar World's "Lick of the Day" iPad app. It's a freebie that comes with a handful of samples; you pick from one of three subscription options to get more licks.
The top part of the screen shows the pro playing the lick and the bottom has two panes showing the frets and a musical scale. The fret/scale is animated to keep time with the pro's video. You can play it at full speed or half speed.
Everything described up to now applies to "video" mode. There's also a "practice" mode that ditches the pro's video and gives you more control over the playback speed. In practice mode you can choose anywhere from 10% to 100% of the original speed. You can also turn looping on (to keep playing the same lick over and over) as well as turn the instrument audio off so that you can just hear yourself while you watch the fret/scale animations.
This is one of those apps that really plays to the strength of the iPad interface. As I mentioned, I'm no guitarist but I'm tempted to sign up for a subscription because it's so fun watching these guys play!