iTunes users frustrated by mislabeled songs and tags will find TuneUp Companion a big help. It's a plug-in that lives side by side with iTunes. To correctly label songs and fix tags, drag songs to TuneUp, which searches through its massive database, looking for matches. It then lets you fix any problems it finds.
In addition, it finds cover art for songs, and it does a remarkable job. It was able to find album art for a more than 30-year-old album now out of print, for example. Dancer with Bruised Knees, by Kate and Anna McGarrigle, if you're curious.) In addition to finding cover art and cleaning songs and tags, it also provides additional artist information, and recommendations from Amazon.
Note: This demo version will clean up 500 songs and handle 50 album covers. If you want to clean an unlimited number of songs and use an unlimited number of covers, you'll need to get the for-pay version, TuneUp Gold, which costs either $20 per year, or $30 to use for a lifetime.