Found the link on slashdot: the US army made their heavyweight BRL-CAD available as open source, for an idea of the possibilities, see this screenshot .

BRL-CAD is a CSG (constructive solid geometry) package, as they really wanted to work with solids: so that they could model penetration of ballistic objects into tanks and so...

What I'm totally not in a position to judge: could this be used for IFC ("information for construction", an building/construction/architecture building model standard with a big CAD background)?

Check out their logo :-)