SLUDGE (standing for Scripting Language for Unhindered Development of a Gaming Environment) is a system by which anyone (within reason) can make an adventure game. It runs in any version of Windows from 95 onwards, in high-res with 16-bit colour.
The engine is free, but the development kit (which includes the compiler) is not. Games created with an unregistered development kit have a dialog box displayed advertising SLUDGE.
Games created with SLUDGE can be freely distributed, also commercially.
Is this information incorrect or incomplete? Let us know!