OSStatus.com
OSStatus.com (via Seth Willits) is a great resource for quickly looking up error codes. It supports not only the traditional OSStatus error codes, but also NSError codes from NSCocoaErrorDomain, kCFErrorDomainCFNetwork, NSPOSIXErrorDomain (errno.h), etc.
Update (2015-10-04): Marco Masser:
This [LaunchBar] action uses osstatus.com to look up information about error codes on Apple’s platforms. This is useful if you’re a developer who wants to look up some error code or constant but you don’t know in which framework or in which header it is defined.
As a bonus, if you have Xcode installed (either as /Applications/Xcode.app or /Applications/Xcode-beta.app), the search results let you browse the related frameworks and header files directly in the respective SDK.
