If you have an Arduino UNO you can re-write the firmware of the USB-to-Serial chip to make it a USB Host.
I doubt that will work. First, host firmware is quite a bit more complicated than function firmware. Second, the Uno would have to provide power to the +5V rail of the USB bus, rather than derive power from it. The way the Uno hardware works, that is not possible.
The Arduino Drum Machine
: MIDI development system with 14-track MIDI drum machine sequencer / groove-box software