Error code lookup tool from Microsoft
When I worked in CSS for MS there were a lot of cool internal tools we could use to help us in our day to day work, and this one tool is great for Microsoft error codes.
Over the years some of these have been made public – the most famous of which is PSSDIAG on codeplex, which was released with some fanfare (at least within the SQL community).
However, there’s one which is (arguably) even more useful as it doesn’t just apply to SQL Server.