A hotels.com price tracker. We've taken the camelcamelcamel concept and applied it to hotels. It turns out that hotel prices change all the time too.
This site no longer exists, but I'm still quite proud of it. HousingTracker compiled housing inventory and price trends during the housing bubble by aggregating online real estate listings. The site was widely cited during the housing bubble and after the Great Recession. Housingtracker existed from 2005 to 2014. Mentions: New York Times, CNN, CalculatedRISK
- 2019: Mars