Osmoscope is a web-based user interface to help with Quality Assurance. The tool is currently in beta.
You can test it at http://osmoscope.openstreetmap.de/ .
The code is available from https://github.com/osmoscope/osmoscope-ui .
Osmoscope can show layers from many sources. Here are some currently available sources. At some point we'll put this list somewhere better, but for the time being feel free to add your sources here.