A lot of the more powerful multi effects units have LFO functionality built in. Essentially you can control any parameter with a configurable LFO. I spent some time messing around with this a bit yesterday, and the most musical thing I could come up with was a non-envelope controlled auto wah. Not terribly interesting. Obviously you can do simple tremolo and vibrato with them, but I'm hard pressed to come up with anything really musical with them. Everything I made sounded "glitchy" for lack of a better word. So what have you got? Share some interesting LFO ideas.