Isometric games are more difficult to design and program than either 3D games or those with a top-down view, and there are far fewer tools and game makers available, which is probably why IsoWorld, though in some aspects obsolete, continues to be sold. IsoWorld is the only tool that I have at least some experience with, but I'm providing links to a few more that I came across.
This is a professional product dating back to 1999 for Windows (may need to run in compatibility mode on XP) and Macintosh (now with full native support for Mac OSX). SpriTec considers it legacy software since about 2004 but has never discontinued it due to popular demand. Demos are available. As of 2011-09-30, the full version costs $39. This is a full license that allows you to sell games you created with IsoWorld.
IsoWorld is designed primarily for RPGs and adventures and supports extensive interaction with the environment in the style of the later Ultima games. Here's a list of features from the website:
24hrgame clock cycle, ambient Brightening/Darkening/Lengthening/Tilting/Receeding shadows.
I played around with it once and noticed a few shortcomings. For example, the inventory is drag and drop and just heaps the item into the bag without snapping them to a grid, so that they will easily cover each over. But on the whole this is a good product.
As of 2013-02-25, the SpriTec website is gone. I don't know if it's gone for good, for the moment I'm leaving the links up.
Tyrone W. Lee has developed this isometric RPG creation system since 1995. You can download a demo. As of 2011-09-30, there are several licenses ranging from $50 to $130, and the version number is 9.0b.
FIFE grew out of a Fallout replacement engine, IanOut. The first release, a pre-alpha for Windows, was in 2006. In January 2010, 0.3.2 was released. Games can be created with Python or C++: This is not really a game maker, but a game creation framework, and certainly no tool for those afraid of using programming languages.