Well, Marilou and Webots are good pieces of software indeed. But they are not very popular (probably just amongs the robotics fans). So, it is very hard to crack these. I spent (and waisted) moths trying to find a crack for Workspace 5 but of course I never succeeded since this Robotics software uses a USB key dongle just to initiate the software. Well, I came across a better a solution, perhaps you'll find it useful as well. Try the NI mechatronics tool for LabVIEW.http://zone.ni.com/devzone/cda/tut/p/id/6183
You require SolidWorks 2007 and LabVIEW 8.2 which are of course the best tools you can find for both, drawing and designing your Robot and controlling your Robotic hardware. In case you just want to simulate your Robotic application, use SolidWorks for your drawings and this LabVIEW Mechatronics tool to virtually "move" your Robot using the LabVIEW programming environment.
I hope this helps!