Abnormalize is a graphical music collection normalizer.
The above definition is technically far from being correct, as Abnormalize knows nothing about normalization, and turns to mp3gain to do the heavy lifting. All it does is provide a nice GTK+ frontend to mp3gain.
The benefit of using Abnormalize instead of plain mp3gain, apart from the fact you don’t need to touch that scary command line, is that it is able to search a directory recursively for songs, so you can run it on a well–organized, deeply–nested music collection without moving any of your files.
Abnormalize can be configured to run mp3gain with your preferred options, although little documentation about this possibility exists at the moment. Abnormalize’s defaults are quite conservative.
To follow, or partecipate to, the development of Abnormalize, check out the information provided in the development section.