TrenchBroom is a cross-platform level editor for the Quake engine. It has a different slightly philosophy than other editors. I believe that the traditional approach, where you work primarily in 2D views, forces you to think in 2D rather than in 3D. That is why this editor does not have 2D views at all - you do all work in one single 3D view.
This software is open source under the GPLv3 license and it is freely available on Windows (XP and newer), Mac OS X (10.6 and newer) and Linux (only from source). Unlike many other free apps, TrenchBroom includes comprehensive documentation. So instead of repeating everything that is already written there, I will keep this short and suggest that you download TrenchBroom and look at the documentation if you want to learn more. You will not be disappointed.