A very crude Linux-only "mod manager".
It works by mounting a custom FUSE file-system on top of a game's directory when the game is launched. All the games files are visible as normal, but additional files from mods can now be added virtually, while retaining a "clean" game install at the root of it all.
The basic idea can be demonstrated with the file structure in testdir/, where
we have the folders gamedir/ and moddir/.
gamedir/
├─ gamefile1.txt
└─ subdir/
├─ gamefile2.txt
└─ subsubdir/
└─ gamefile3.txt
moddir/
├─ modfile1.txt
└─ subdir/
└─ modfile2.txt
m3 can then "overlay" the moddir on top of gamedir, giving us
gamedir/
├─ gamefile1.txt
├─ modfile1.txt
└─ subdir/
├─ gamefile2.txt
├─ modfile2.txt
└─ subsubdir/
└─ gamefile3.txt
This effect could also be achieved by directly using overlayfs rather than
implementing our own poor man's overlayfs, but I just felt like using FUSE
directly.