If you have any interest in slap bass, you've probably heard of MarloweDK, who has a massive following on YouTube and guests over at notreble.com.
For the last couple of weeks I've been using his iOS app for learning slap bass, Beginning Slap Bass
, and it is awesome
Video lessons, with tab synced up, adjustable speed playback of riffs with adjustable bass/drum/click volumes. Super great.