I would like to edit the metadata of several mp3s at once (say Album, Artists etc.). What is a quick and easy way to do that? I have Windows, OS X and Ubuntu available, so I don't mind using any of these OSes, as long as it's easy and quick.
I think Mp3tag is the best for your need. It is a powerful and yet easy-to-use tool to edit metadata of common audio formats where it supports ID3v1, ID3v2.3, ID3v2.4, iTunes MP4, WMA, Vorbis Comments and APE Tags.
I use mp3info on Linux. On the command line you can quickly update the tags, like mp3info -a "artist" -t "title" -l "album" -y year file(s).