XreaL's reworked version of radiant apparently can use Blender objects. That is an important step to minimize redundant work. So many components are there, but in a disarray.
Looking at Nexuiz again, I think they have done a marvellous job of releasing maps at http://maps.nexuizninjaz.com/.
Dooms3 maps can be found at http://doom3.filefront.com/ and http://www.map-factory.org/doom-3
And, http://www.qeradiant.com with its offspring http://darkradiant.sourceforge.net are fantastic too.
So, even a great deal of artwork is there, almost like cherries hanging from a tree.