People like me using computers hold a huge collection of music on their PC, and a time comes when we feel that we should have had tagged all the music files while we were procuring them or ripping them. I personally have more than 4500 songs and since they are not tagged properly, my music player has created an unmanageable directory structure. Cleaning up the entire mess would take around an year. It would require listening to each and every song and then searching for valid info and album art on the internet to tag it. I had given up on my library.
But today, I came across an amazing application , which works as an add-on with iTunes. If we process the iTunes media library through this application, it automatically finds the data relevant to that particular song and tags it accordingly. No human intervention is required. It even searches for suitable album art and adds it to the file. It also searches for duplicate files and informs you for the action. The application that I described here is TuneUp companion. Check out the trial version before buying.
But I am sure that after trying out the application, you will surely buy it.