Comment 5 for bug 221433

This is a known issue with some of the default screenlets.
This thread gives details on what needs to be changed for these memory leaks to stop - http://forum.compiz-fusion.org/showthread.php?s=0d0e639fecc3463c25ae5c9b2143d40a&t=3576
I've applied the fix to pidgin-screenlet myself and after a couple of days there really doesn't seem to be any leaks...

As far as I know there's no community effort to fix all the screenlets (even though the thread is from Aug 07 :)) but this fix is fairly easy to apply anyway... if you need more details feel free to message me.