A working version of the filtering box (loading might need refactoring)
It's already a problem that there are to many gadgets to quickly locate the ones you want. Sections help but they are not very strict and meaningful.
Also more gadgets for even small stuff could be added be then user would never find them... We (on pl.wiki) even had a questionnaire to remove the least used gadgets.
So. I think a simple filter input would help solve this issues.
I'm attaching an add-on script that allows filtering as the user types leaving sections in place. User can filter by any part of a word in description and even by two parts of a word in any order ("nav po" should find Navigation popups). I've tested this on English and Polish Wikipedia so hopefully it should work without problems, but you might want to put GadgetFilter.init() in some more appropriate place.