Clive, there is a free 3D solid modelling package called OpenSCAD. I've not used it, but if it's anything like their website says, it could be OK. They say it imports and exports files for rapid prototyping (& 3D printing) so I'd say it also translates to & from .stp, .sat, parasolid, etc.
http://www.openscad.org/index.html
Depending on the client I am working for at the time I use Inventor, Alias, SolidWorks, Catia or SolidEdge for 3D, & typically AutoCAD for 2D stuff like P&ID's, schematic layouts, etc. All my work is mechanical design. I reckon I've used 20+ packages since I started using CAD (30 years ago).
If you do give OpenSCAD a try I'd be interested in hearing what it's like.
Bit different to the old days of mylar film and ink pens.