I'm moving my patched and improved version of Pascal Frey's medit software from libigl/external to its own github repository. The new version still depends on libigl and AntTweakbar. There's a matlab wrapper for it in my gptoolbox.
This tool has been essential to my research on tetrahedral meshes. There are more advanced tet-mesh, 3d-slicing visualization tools around, but this one is straightforward and open-source. I've made some modifications to make it easier to visualization data and make more complicated slices (hot-dog view!). The input is the same .mesh tet mesh format used by TetGen.