Error Lookup is a free tool which converts cryptic PC error codes into a (hopefully) more meaningful text description.
The program works with regular Windows codes, DirectX, NTSTATUS (Native API), WinInet, Device Manager, STOP (BSOD), RAS/VPN and Ip Helper library error codes.
While this might sound complex, it really isn't. Just type whatever you need into the "Error Code" box and Error Lookup immediately displays any matching descriptions (you don't even have to press Enter).
The text can be copied to the clipboard, convenient if you need to pass it to someone else, or maybe search for it online.
If you're only interested in error codes of a particular type (BSOD, say) then Error Lookup can be configured to display those only. The program can also be set "always on top" of other windows for more convenient access.
Version 2.3 (changelog)
added window sizing
increased startup speed
removed settings dialog (do not needed anymore)
removed internal error codes
updated facility list (sdk v10)
updated to the latest sdk
In most cases a web search will give everything you need on a particular code. But Error Lookup also works offline, and supports a wide range of error types, so it's probably worth keeping a copy around.