eBay
As featured on Discchord and Synthopia: This IO Dock has been enhanced to include 3 USB ports - think of it as an IO Dock married with a Camera Connection Kit and a USB hub. It is possible to connect USB controllers and class-compliant interfaces to this IO dock - which will still work as one of the best Audio and MIDI interfaces, and charge the iPad. Please note that normal, unmodified IO Docks only work as USB slaves - i.e., you cannot connect USB controllers like keyboards or additional audio interfaces to it.

Let's be blunt: To achieve this, I hacked it. This IO Dock has been the first of its kind - which brought us modest fame on the internets; to find out how it was done, and what the guys from Alesis have to say to this, have a look at the Youtube video: https://www.youtube.com/watch?v=SLYteU5r8tE . It has been running since January 2013 now, and the modification has been replicated by others.

So get a piece of modding history - and a great piece of equipment - and bid on this dock.  Come on, buy it - and break my heart.

Apart from the modification, it is a standard IO Dock for iPads 1/2/3 (with 30-pin connector): No functionality was lost through the modification.  Newer iPads (4th gen, iPad Air) can be used with the Lightning-to-legacy adapter but won't fit the frame.  (Which is the reason why I'm selling it: I've got an iPad Air now, and I'm using a Novation X-Station synth as my interface. Not as elegant, but does the job.)