Beta 4 of the open-source Doom level editor Why has been released. According to the page:
This version has a few minor tweaks and one major new feature: the super easy, ultra efficient sector splitter tool. It is so simple that I can explain it with just two images. First you select the vertices that are the basis of the split. Then just select the splitter from the pop-up menu and you're done. All the crossings in between are calculated and added automatically and proper sidedef references are maintained throughout. The tool can handle anything, such as donut sectors, areas that are outside sectors etc.
Why works on Windows and Linux machines, and I must mention that that sector-splitting tool looks extremely cool.