If you've spent any time whatsoever in SketchUp, there's a better-than-average chance you'll have a ball within Autodesk 123D. The new Windows-based program -- which quietly slid into beta a couple of weeks ago -- is a gratis, stripped-down version of Autodesk's famed Inventor application, but it still offers a plethora of design tools for something that's being given away free of charge. We're told that just the basics are here, and sadly, it's not open source; that said, it does have an option for exporting to STL, and you will find support for assemblies and constraints. Eager to toss something else in your 3D modeling toolkit? Hit the links below to get started, and feel free to check your wallet at the door.