Basically, the plug-in powers up cover art discovery while cleaning up metatags, automatically discovering contextually-related Web content, and sniffs out upcoming concert information. I haven't yet had a chance to check out the Mac version yet, but keep in mind that it's a beta and there is likely some serious debugging needed.
In fact, the bugs aren't likely--they definitely exist. In the words TuneUp's media relations guru Andrew Kippen, the core features work but, "there will be bugs." That may not matter so much if you're the kind of person who likes to get in on the ground floor--TuneUp Media has given Download.com 100 invites to check out the Mac beta.
Send an email to firstname.lastname@example.org with your name as the only thing on the first line in the body of your message, and let us know in the comments here what you think of it.