Some popular keys
Reports
Reports show the tag data from different angles. They often bring together data from several sources in interesting ways. Some of the reports can help with finding specific errors.
About
OpenStreetMap uses tags of the form key=value to add meaning to geographic objects. Taginfo collects information about these tags from several sources to help you understand what they mean and how they are used.
International
This is the Swiss Taginfo instance. It shows only tag information of OSM data within Switzerland. It is updated daily.