The iPad Camera Connection Kit

 
Transient

Don't let it's name fool you, this little dongle is much more than a device for connecting your digital camera, it's a fully fledged USB port for your iPad.

I often use this to plug  my Roland piano keyboard into my iPad for some Midi action in GarageBand. Many USB mixers and microphones are compatible too. If you need more ports or extra power for those phantom powered mics, you can plug in a powered USB hub.

If it's a plug-and-play USB device, chances are good that it will work on your iPad with the Camera Connection Kit. I suppose you could even plug in a USB keyboard if you wanted (not a mouse though, because using a mouse or trackpad on a tablet would just be stupid).