The Meaning of Life, the Universe, and Everything.
My favourite is InvTweaks. It does a bunch of stuff that I don't use, but the one thing it does that I really love: If you break a tool/weapon or burn through a stack of items and have the equivalent in your inventory, it auto equipes it fr you. Saves tons of time branch mining; building; farming. It does all sorts of other auto sorting stuff, but I've never really looked into that. The feature above is enough for me.