With Google SketchUp it is easy to make 3D models and even quite complicated 3D scenes. Libraries of optomechanical components can speed up the process of an optomechanical setup documentation, presentation or prototyping. Newport offers 170 product models via Google’s online SketchUp Warehouse and there are few other libraries such as Andy’s (mainly Thorlabs products) and Pakl’s libraries.
The biggest source of free 3D models for electronics is probably RS-ONLINE. They offer a comprehensive 3D model library (look for 3D logo in their online market) supporting many common CAD formats. The modelling itself can be done in their own free DesignSpark PCB software or the PCB Converter tool can be used to convert Intermediate Data Format (IDF) files to COLLADA format for Google SketchUp. 
With the PCB Converter tool it is possible to import a 3D representation of PCB design models (from applications such as DesignSpark PCB) into Google SketchUp with a simple drag and drop conversion process. Few collections of 3D electronic component models are available via Google’s online SketchUp Warehouse: Electronic Parts And Circuits, Electronic Components, Darlington’s Electronic Components, Electronic Components etc.
Google SketchUp runs on Wine under Linux.
Update 17. 4. 2013: 3D Models of SparkFun Parts.
