*edit*
Here are some links. Some of these have short learning curve while other demands that the user knows C++ etc.
- AGS - Engine for making 3rd person point and click adventures like Monkey Island.
- RPG Maker 2000 - Engine for creating simple (Final Fantasy clones?) RPGs.
- RPG Maker 2003 - Engine for creating more advanced RPGs than RPG Maker 2000.
- Ianout - Engine for making Fallout clones
- IrrAdventure - Engine for making Myst clones
- OGRE - 3D graphics and physics library suitable for RPGs, adventures etc.
- ErisMUD - A MUD engine
- J.U.R.P.E - A dungeon crawler engine
- Worldforge - Engine for 3D games.
- Panda3D - Another 3D engine (claims to have a short learning curve)
- OpenGL - A great 3D graphics library
- SDL - A multimedia library
- ZClassic - Engine/Editor for making Zelda clones
