Here are five that I found:
DesignSpark Mechanical
FreeCAD
Blender
OpenCascade
NETGEN
I've only used DesignSpark Mechanical, FreeCAD, and Blender. DesignSpark is the best and is built on the SpaceClaim kernel.
It appears that OpenCascade and FreeCAD may be built on the same core/kernel judging by the screenshots.