Windows: When something goes wrong in Windows, you may only receive an unintelligible error code and no explanation. Instead of sifting through Google results, portable utility ErrMsg will look up the code for you.
Windows does sometimes give you a short explanation of the code, but when your friend calls you over to their house with a blue screen of death and an error 0x80070570, it's nice to have this handy tool available — just start it up, punch in the error code, and it will tell you that their directory is corrupted, allowing you to move on to actually solving the problem. Sure, with some more obscure errors, your next step will still be Googling what the next step is, but when it's something as simple as a file being read-only, you usually know what you need to do to fix it — and perhaps offline, if the problem involves a net connection. It's a great little time saver, and softens the blow of Windows' often cryptic ways.
ErrMsg is a free download, Windows only.