Hi,
You mean xBIM -go to www.openbim.org
eXtensible Building Information Modelling - although my personal interpretation for BIM would be Building Information Management.
There is a lot of BUM around too :) (Bloody Ugly Modelling)
Marcel
Just to add to the last post. You don't need ArchiCAD to view .bimx documents!
This format is only applicable to ArchiCAD, which allows you to create 3D models in an .bimx format. The file can be viewed with viewer applications (on Mac OS X, Apple iOS and Windows)
BIMx is an interactive 3D presentation tool.