Four suggestions of apps I've used in the past. Only one of them is free (and go figure that one isn't exactly iTunes-friendly, not easily anyways).
Pollux - Pollux was featured on net@nite (Amber MacArthur and Leo Laporte), and subsequently died off for months due to illegally using various APIs from services like (IIRC) MusicIP, GraceNote, and other big name commercial music vendors. After negotiating agreements with companies that could provide plenty of helpful information, they re-launched the product with a 20 song tag trial, and unlimited tagging for $10. I believe this is the cheapest app that integrates as well as it does into iTunes.
It's UI is geared heavily towards albums, wherein you select all the tracks in an album, then click the Pollux icon in the menu bar, and then click "Tag". It goes about it's business pulling information, cover art, and more (where possible), then does it's work. Updating your music with pretty information.
Afterward, you won't re-live this terrible memory.
TuneUp - TuneUp is similar to Pollux, except it's a bit... obnoxious. It runs as a separate app that shrinks iTunes and places itself it the far right of it, like an even more annoying sidebar than Ping. Again, choose your song(s), tell it to "Clean Up", and off it goes searching the internet for all the same information you want to make your information clean as a whistle.
You can buy TuneUp for $20 for a year (warning: automatically renewing), or $30 for life.
Song Genie - I'm actually quite a big fan of the Song Genie and Cover Scout duo, perhaps solely because it's UI/UX is ahead of it's curve. You open the app and you get to work. TuneUp is too small and has too few options for checking the details of the data, and Pollux is one-shot (though it allows you to rollback the last action, thankfully).
Song Genie uses fingerprinting to identify the song and pull in data. It works very nicely with the iTunes Library, or even any arbitrary folder on your drive. It "rates" your data, and can fill in the gaps marvelously well (lyrics where you had none previously despite having plenty of other information). With a large, attractive, simple interface, it's actually pretty easy to whip through tagging in a quick manner (subject to the CPU speed of fingerprinting, and your internet connection for searching). Unfortunately, this dream team duo is expensive. $30 for Song Genie, and $40 for Cover Scout.
I note also their site says "Your Special Price", so there may be some sort of active deal at the time of writing.
MusicBrainz Picard - Picard is in a class all it's own. It doesn't integrate alongside iTunes well, and it's UX flow is god awful. However, if you want absolute control in tagging, nearly the experience of manually writing tags, with just a bit of assistance to make sure you're doing it right, Picard cannot be beat. (And yes, this is a good thing.)
Picard is not at all hands off, unlike the majority use cases of the rest of the apps, but this makes it awfully better for more niché releases, and bonus: If it can't find something, do the work and contribute back to the MusicBrainz community! We'd be glad to have you as long as you follow the rules and contribute responsibly.
Picard is free, no ads, no account required unless you want to contribute, and I highly suggest you use the Intel-only -NX version, unless of course you don't have an Intel CPU Mac.