I know somebody has already done this but they were being all bitchy about not telling how they did it. I, however, will happily tell you how it works. The whole this is an info_particle system, stored in the little orange box (hah, pun). The info_particle_system is set to trace all around it with a trace collision property.
Here's a download link if you want to play around with it. If you do it in GMod you don't need the particle_manifest.txt file.
