I can confirm what stevehero is writing. You can use it in your commercial company for free.
For example if you're Amazon.com and want to create your own music streaming service, you can use Mp3tag for free to build your music database. However, you cannot – as part of your service – offer Mp3tag for download (but you can link to Mp3tag's download page).
I'd greatly encourage you (and anyone else who is reading it) to consider donating if Mp3tag is of (business-) value to you. I think users from a commercial environment have the greatest possibilities for giving something back, but are most of the time bound to keeping the costs as low as possible. I'd love to see some change here.
Kind regards
– Florian