As someone who never really got the hang of sketchup, this looks interesting. Free solid modeling tool, billed as a stripped down version of Inventor and keeping feature such as assemblies, constraints and .STL export.

Downloading a copy now.

http://www.123dapp.com/downloads

Having recently developed an interest in CAD and modelling (i.e. industrial design tools not standing around in a bikini). I found this handy video on CadJunkie that runs over some of the different approaches some of the different packages take (Subdivision, NURBS surfacing and solid modelling). Looking at some of the awesome CAD renders on here i'm sure many members are already quite familiar with this, but for anyone who like me hasn't really touched CAD since the early 90s this is really cool stuff.