If you're staring down a cryptic error code, ErrorGoblin can help you decipher what it means, give you additional information to further your search, and even provide a solution for the particular error you're encountering.
ErrorGoblin supports Windows, Mac and HTTP errors — there is no radio button to select only HTTP error codes, but it will return information if you plug one in. The search results show information for the error code you searched for plus similar codes in case there wasn't a perfect match. Error codes with available solutions have the solution displayed under the additional information section of the error code readout.
ErrorGoblin is a free resource and requires no registration. Have a favourite tool for divining error codes? Let's hear about it in the comments.