Linux

I use QMP3Gain. It actually changes the bits in the file so the files work with every app and player without needing to rely on tags. However, It also adds tags to the file which let you undo the changes later, if needed (it's lossless).

It has two modes. You can modify all the files in an album equally so that they play at the same relative volume. I guess the loudest song is set to your target volume and then the rest are adjusted relative to that. It's great because it doesn't ruin the flow of albums whose tracks connect seamlessly. Or you can modify tracks irrespective of other tracks, which is good for random singles you own.

The result is, songs in your entire library all sound more or less the same volume. The exception being that quiet tracks from certain albums will still be quiet.

You can drag every full album you own into the UI, and do them all at once in album mode. It works based on tags. Then do the same with all singles you own in track mode.

It defaults to 89.0 dB, but I prefer to use 95 dB because some devices just don't have enough volume. A tiny bit of clipping is imperceptible because decoders account for it. Many of your current MP3s already have clipping, and I'm sure you haven't noticed. So don't worry if you see red "Y"s in the clipping column.

It uses the ReplayGain algorithm. Once in a while there's a track that it just doesn't get right. A certain single will just come out too loud or quiet and needs a different dB value than everything else. Out of the thousands of MP3s I have maybe five files have been like this.