I was wondering if there was any good software 'out there' for organizing files with some kind of tags or metadata. For example, a file might be tagged "additive number theory" and "combinatorics" and might have a title, author, and year as metadata. Ideally all could be searched and a list of tags could be displayed, either by frequency of use or categorically. Data could be stored by the program directly or it could be stored in the files themselves (if, like pdf/jpg/png/xls/doc/mp3, they have the capability). Free would be nice, but I wouldn't object to buying software that did this properly.