A reader pointed out (many moons ago – sorry!) Crystal Architect, an IDE for creating Linux games:
CrystalArchitect is a software that helps you create your 3D worlds in a fluent and cooperative way.
It provides all the tools to manage your assets, build scenes, make use of entity systems, add logic, live program your world, etc. Of course, you can also do this in a cooperative way : CrystalArchitect supports networked editing, which is a blessing for teamwork.
On top of that, CrystalArchitect supports as many 3d engines as you want, with currently an emphasis on Crystal Space and CEL.