Protoreto is a GPL-licensed tool to create prototypes of graphical user interfaces (GUIs).
NOTE: This project is not actively maintained at the moment. If you'd like to continue developing Protoreto, please send an e-mail to: wikipedian@users.sf.net
Whatzit provides a system for using error and status conditions much more rich than the canonical 'zero means success, anything else is situational' mechanism. You can tell the severity of a condition and where it originated by looking at its value.