I've used Easytag in the past and found it perfect for my needs, but I don't believe it can run portably, for example from Dropbox. Is there a program that a) will let me edit ID3 tags of mp3 files, preferably multiple tags (for example from the same album), and b) runs from a single folder, without needing admin permissions or needing to install anything in the registry, system folders, etc.
As you are happy with Easytag, and the only thing you miss is portability: there's Easytag portable available matching all your requirements perfectly. Enjoy!