CoMET is the Crimson Fields Map Editing Tool. Please keep in mind that this is still a work in progress (and much more so than the game itself). Currently, the editor is considered to be in alpha state.
Most basic features are implemented. You can, for example, create new maps, load, edit and save them, so it can already be a great help when designing missions. One of the major limitations is that you can only create single-line messages in plain ASCII. As a result, CoMET is for now only really useful when used together with cfed, which it will eventually replace. You can do the map layout with CoMET, export the result as a text file, and add the missing bits by hand (see the cfed documentation for details on how to do this).
CoMET can read the binary level files created with cfed, and you can make modifications to them. Even settings not yet supported by CoMET should remain intact during this procedure.