One of the big challenge of building robots is to come up with good electromechanical designs. Google Sketchup (now at version 8 ) is free and, as far as 3D software go, very easy-to-use with many features permitting coming up with useful plans for your next robot. Most useful for the 3D-challenged however is the 3D Warehouse with user-contributed models. You can, for example, find a model of the Arduino Dueminalove and a range of sensors (such as the Parallax Ping or the Sharp IR sensor) in addition to parts such as servomotor and RC chassis. These can be directly inserted into your design by simply going to File > 3D Warehouse > Get Models.
The downside is that you’ll need to buy the pricey pro version (500$USD) to get the export features – which will be handy when you when to get your mechanical parts produced. But if you’re a Xinchejian member and working on a fun project, feel free to ask me to export your model to one of the many formats available.
(Disclosure: I work for the company that makes Sketchup, however I don’t currently have any relationship with the team.)