Last night I put the finishing touches on an RjDj scene that I've been working on in my spare time called GhostWave. It's a dark algorithmic drum & bass generator which lets you hum the bass lines. If you have the latest version of the RjDj single (free!) installed on your iPhone or iPod Touch, you can simply visit this page from safari on the phone and click the following link with your finger:
By itself it does very little but make randomly raw semi-melodic noise. Shake it quite hard a few times to start some hi-hats tickering, and then hum a bass line into the mic. To record and loop your hummed bassline, put your finger on the little ghost as you hum your melody in time to the hats. Let go when you're done and the melody should loop. Once you're done, shake it vigorously a few more times to bring in the beat. After listening for a while the beats will fade, but you can bring them back by shaking some more, and you can continue to press the little ghost to record more melody loops.
Have fun!