I can't remember who asked this question at the Geekzone thing ... but here's the link:
Music Brainz

MusicBrainz is a community music metadatabase that attempts to create a comprehensive music information site. You can use the MusicBrainz data either by browsing this web site, or you can access the data from a client program — for example, a CD player program can use MusicBrainz to identify CDs and provide information about the CD, about the artist or about related information. You can also use the MusicBrainz Tagger to automatically identify and clean up the metadata tags in your MP3 collections


It'll tag those files that you can't remember what they were - as well as fixing the file structure and a few other features i don't use.
all the best media library apps have musicbrainz lookup builtin, like my favourite : AmaroK