I can't recall how long I have used MP3Gain, but it's fantastic. I use it for CDs in my car and across any/all music players I've owned. You also don't need to use any kind of leveling in your library software as they will come out they same.
MP3Gain can do per track and per album leveling. So, I typically use 94db for my tracks, and each track will be centered near that. But, when working with orchestral music (or when the album has an entire theme with varying sound levels as part of it) I will use album gain to keep the same flow, but the loudest portion of the loudest track won't be obnoxious when compared to other tracks.
You can also get an updated executable that works with MP3 and AAC files. I was using AAC files, until I got this and had to go back to MP3 due to lack of support (hint, hint). With either format, you can always dump the settings or change them and not affect the actual encoding of the file, so it isn't permanent by any means but it is enforced across all software and devices that can understand and play them.