Triage is a piece of free (as in freedom), open source software for keeping track of a large number of files by using tags and named fields. Tags are arbitrary pieces of text, and named fields are like tags that can have other text associated with them, and this other text can be searched. In fact, almost everything about a file can be searched with Triage except files' contents.

NOTE: Triage hasn't officially been released yet, even though it's available for download on this site.