[These visualizations](http://www.win.tue.nl/~cwillems/) of vessel movements, i.e., the movements of ships near port, demonstrate an interesting technique for showing particle movements. From the abstract:
> Our visualization is based on density fields that are derived from convolution of the dynamic vessel positions with a kernel. The density fields are shown as illuminated height maps. Combination of two fields, with a large and small kernel provides overview and detail. A large kernel provides an overview of area usage revealing vessel highways. Details of speed variations of individual vessels are shown with a small kernel, highlighting anchoring zones where multiple vessels stop. Besides for maritime applications we expect that this approach is useful for the visualization of moving object data in general.
The [original paper](http://www.win.tue.nl/~cwillems/public/eurovis09.pdf) is also available. Link via [visual complexity](http://www.visualcomplexity.com/vc/project.cfm?id=705).