I've spent several hours trying to complete my music library with the correct album covers, including various tracks I really don't remember where I got. I'm sure some of them are illegally copied from somewhere, but the majority are ripped from physical CDs that I own. (I have a whole bunch of old vinyl albums which I haven't digitalized yet, though.)
Mp3Tag has been an invaluable help; it can search freedb given a set of tracks, and it can also search Amazon.com. There is also a menu option for searching discogs.com, but that never seems to work. I'm not sure why.
Several hundred tracks did not have any tag info at all. I had to go purely by file and directory names. Very weird, I'm sure that when I ripped them, they looked good. My programmer brain wanted to start disecting the files to figure out if the files contained any clues to which program I used to rip them, but the rational part of my brain said that that was a waste of time. Which it surely was. I'm still curious about how it really happened. After getting the album and song names correct, Mp3Tag could fix the rest.
I still have problems with a couple of albums. I borrowed a CD from my sister (which I still think I haven't returned -- I'm sure she's forgotten): "Pops by Gershwin" with John Williams and the Boston Pops Orchestra. I can't find any information about the album at all. Even Google lets me down.
The OCD part of my brain tells me that I have to keep going until there are no more unknown album covers, while the other not-so-OCD part protests and would rather play another level of GTA:IV. We'll see which part wins.