I’ve been using J. River’s Media Center since, oh, I dunno, maybe version 7. It was called Media Jukebox then, I think. Version 10’s in beta now and I’m very pleased with it. There are a lot of free media managers and taggers out there and I’m always looking. Two that have recently come to my attention are The Godfather and Media Tagger. Doesn’t look like either of them maintain a database like the J. River products but they still look like they can be handy.
Two others that I’ve looked at and am pretty happy with are ID3-TagIT and Mp3 Book Helper.