has somebody a good hint for achieving a pseudo stereo effect for mono signals? i tried with fft, distributing the bins randomly to l and r, but i couldn't hear any effect.
i'd be thankful for every suggestion.

It's not pd but maybe these links might help:

http://dallashodgson.info/articles/OpenAmbienceProject/index.htm
http://www.soundonsound.com/sos/jan07/articles/pcmusician_0107.htm

thanks domien, that helps. pretty easy, i didn't think about only to delay one side a bit. sometimes i'm so dumb....

You could also send the signal to 2 different processes, 1 locut 1 hicut. The cutoff freq should give you some control of the effect intensity. Plus a non mono compatible tiny delay on one side, of course but watch out for off-phase weirdness.