I was looking for a hammer editor replacement, but the only things I've found so far are for quake/goldsrc. Obviously unimportant as I like the hammer editor (when it works), but the customization options from an editor like Sledge would be nice to have. Or just less crashing.
Only one I know of is what Vesper/Herbius was working on but I think it's for his education and it's not for public use. There's always Microbrush for blocking out brushes.