TopOpt is developed by Technical University of Denmark. The 2D version was online by 1999 and since then the online tool has been used for more than 180.000 times. Its 3D version was online by 2004 and is used for more than 7000 times by now (2011). The applets are free to use and their purpose is to promote Topology Optimization to masses. And we also for the first time had the opportunity to use a topology optimization software with their applets! As you can guess the applets have rather limited capabilities as they are not designed as commercial tools. The number of elements that can be used is limited and geometry has to be defined by very basic box tools. The 3D applet uses voxels so meshing is not necessary.
2D optimization result
3D result from TopOpt
Top opt is ideal for introduction to Topology Optimization, it has tools like defining voids, filled regions, three loadcases and it is quite accurate too. So if you are new to Topology Optimization, go on and check their web site. All you have to do is place some forces and supports, and watch the structure take shape.