Looking for isometric game engines, I found three great projects: FFilmation, As3isolib and OpenSpace.

FFilmation (MIT License):

• Have a programming interface as small and easy as possible, no matter how complex the internal code is;
• Rendering performance is a major concern when designing all the algorythms;
• Graphic designers are able to work on the application’s ( game or not ) environments without any programming skills, visually, and with almost immediate previews of what they are doing.

As3isolib (MIT License):

• Simple scene creation;
• 3 primitive types;
• Base class for displaying user-created conten;
• Plenty of styling option on vector based primitive;
• Integrates well with a variety of tween engine;
• Improved 3D isometric positional sortin;
• Scene & view renderer classes geared towards performance optimizatio;
• Performance caching of static and animated sprite asset.

OpenSpace (commercial, part of the SmartFoxServer platform for multiplayer games):

• Engine and framework for rapid development of multi-user virtual worlds and MMO communities;
• Customizable tiles' aspect ratio;
• Tiles Elevation
• 2D objects in a 2.5D world
• Backgrounds and foregrounds
• Scrolling
• Advanced pathfinding
• Powerful avatar management
• Advanced map interaction system
• Exhaustive documentation
• Complete Map Editor

If you were planning to develop an isometric game, you just found your first step.