If you're a musician, aspiring beatmaster, or just really love dinking around with a digital keyboard, I have great news: FL Studio Mobile is finally here for Android. Device compatibility seems decent, but I don't have too many tablets around to check with - I do know it's compatible with the Nexus 7, and a fair number of recent Android phones (oddly, not the HTC One).
For the rest of you, yes, you read the title correctly - FL Studio Mobile costs $20. And there's a reason for that: it's probably the most robust mobile music project and synthesizer / beat creation app this side of Garage Band for iOS, and as far as I am aware, nothing on Android currently comes remotely close to this. The big sell is really to those who use the desktop version - FL Studio, which has been around for over fifteen years. Impressive.
The mobile version allows you to work alongside the desktop version with a high degree of cross-compatibility - all the included instruments have FL Studio desktop counterparts, so they work natively in both the mobile and desktop versions of the software.
133 instruments are included in FL Studio Mobile, including a virtual piano-keyboard and drum pads, should you want to lay your beat down the old-fashioned way. There are a ton of features here, and I'm not going to pretend to understand most of them, so if you're interested in this app, you probably already know what you're getting into. A full feature list is available on Image Line's website, here.