VMFSketch

VMFSketch is a rapid prototyping tool for maps made with Valve's Source Engine.

Quickly sketch a floor plan, add walls and ramps, then open in Hammer.

Fast
VMFSketch makes it easy to move, resize, and create brushes, and the focus on keyboard shortcuts means most actions are only a keypress away
Robust
VMFSketch has no known crash bugs (yet), and doesn't generate invalid geometry.
Easy
VMFSketch lets you get to what you want to do: layout a map. The layer system lets you move and modify collections of brushes with ease.

Current Version: Alpha 1: 2017-4-1

Initial release! Doing a kind of Sean Barett thing (stb_voxel_render.h, anyone?) and releasing a serious project on April Fools' day.
Alpha 1 is complete with regards to rectangular brush editing, but not much else. In this version you can create, move, and resize brushes; edit vertices, create slopes, specify and modify layers; save and load "mapsketch" files (I might change the filename for the next version), and finally, export to a vmf file opened in hammer.
As far as I know, VMFSketch doesn't generate invalid geometry or have any crash bugs. If you find one, say something!
Roadmap: The next thing I'm working on is undo functionality, which requires reworking a lot of the internal editing components. This has a secondary benefit: by modularizing the modes, tools, and actions, it becomes very easy to create more and load extensions. The next version should also feature some more layer properties, such as rotation around the Z axis.