OpenStreetMap
I started using the Keep Right data consistency checks for OSM. You can find the start page here:
This is a great tool for checking your own work and that of others. For a beginner like me it's a wonderful training tool. It was a little bit daunting at first, but once I got the hang of it I managed to correct (I hope) lots of errors that I had introduced to the map.
If you have the Remote plugin installed for JOSM, your clean up task will be a lot easier:
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