I am working on a little level editor/3D modeler that is not meant to do all the stuff a full editor like Hammer/Radiant/Sledge/Trenchbroom does, but augment the level editing process a bit. I'm embracing the math of brushes (and now also curved patch meshes) and trying to build a powerful tool around it. The thing is programmable out of the box without third-party tools and supports a variety of level formats (HL1 .map, HL2 .vmf, Q3 .map). While my focus is very narrow at the moment (mostly just the bare geometry), I hope that someone who got a bit used to it can brush out their level a lot faster with it than with other editors.
Here is a video of it in use: http://www.youtube.com/watch?v=wjjB8MLjvJ4
And a video from today showing my first steps with curved patch meshes: http://www.youtube.com/watch?v=HhBy8XjdvG8
An old and a new pic (new pic showing the export to radiant): At the moment, I can not offer a polished 1.0.0 release of it yet as my todo list is still very crowded... but I'm actively working on this every other day.
Work in progress versions can be found here.
The editor is a standard Windows application and uses OpenGL for rendering.
I'll share updates in this thread.