Some of you might be familiar with the Hammer plugin Propper, which lets you export a VMF as a Source model. Or it did, before the switch to VPK broke it and the creator mysteriously vanished. Currently, a new open-source project has been started that's meant to replace Propper, called JProp. It's going to be Java based, so it will also work in MacOS and Linux in case Valve ever releases a level editor for either of those systems. I know some of you have noodled around with third-party tools that either read or write VMF data before, so if you think you might be of aid to this project, consider dropping the creator a line.