SpaceClaim has earned a very good name as a direct modeling software and also has an easy and intuitive way to do assembly modeling. AR-CAD has developed an addin inside SpaceClaim to perform motion and dynamic simulation of a mechanism or an assembly.
A basic overview of SC-Motion 2012 (version) is given below in the form of a video tutorial at https://www.youtube.com/watch?v=bLKIBMHgn1U
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SC-Motion 2012 is available for 99 USD. More details at http://www.ar-cad.com/sc-motion/index.html
Other SC-Motion tutorials are at http://www.ar-cad.com/sc-motion/tutorials/index.html