OpenHeatMap
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For detailed help and information, go to http://wiki.github.com/petewarden/openheatmap/

OpenHeatMap is a system for creating, displaying and interacting with data visualizations based on geographic data. Written by Pete Warden <pete@petewarden.com>, all code is licensed under the GPL V3. For more details on the license see the included gpl.txt file or go to http://www.gnu.org/licenses/

It's not part of the OpenStreetMap project, but it does use the OSM XML format for storing geographic data.