How about using tags (ie keywords) instead of categories? I have this pet theory that categories -> tags -> search is the sequence of information architectures aligned for fitting bigger and more dynamic data pools. Categories are the most restricting, but the fastest (because of good discoverability) - search the most powerfull, but slower as you need to think up the query terms and tags somewhere in between. By the way I am developing a unified tag/search engine for bookmarks (a la del.icio.us but more integrated) and I am just thinking about the possibility to use it for other things like emails etc. (hmm perl modules?).