For AAC/M4A there's also https://github.com/nu774/m4acut, which can split MPEG-4 based AAC audio and preserves gapless playback. (While mp3DirectCut can read AAC in MPEG-4 containers, it only ever spits back out the raw AAC stream, which lacks the necessary tagging for gapless playback.)

I've found handy for splitting up the occasional concert regording off Youtube back into individual tracks, and things like that…