This site contains excellent animations based on US FAA flight data, as noted on the OSM mailing list over the weekend by Andrew.
They are QuickTime movies, so Linux users will need to install something like Mplayer and the mozilla-maplyer plugin to view them.
[...] I blogged about Aaron’s flight path animations a while back. In June this year, I met Aaron and some of the other guys from the Yahoo Design Innovation Team at WhereCamp, who were showing off their animations. One that I particularly liked pulled data from the Yahoo! traffic API and created a frantic visualisation of accidents accross the US. [...]