The Forge is the creation of Benji Adams - "the Professor" - a keen fan of AI bots. It provides a simple, browser based interface that allows anyone to design their own template based chatbot with no knowledge of programming or any technical details such as AIML. You can also, of course, chat with the many thousands of bots created by others.
The Forge also provides some special features not found in more standard AI bot systems such as an emotion engine and automatic response generation for certain unrecognised input. The result is that anyone can get a bot up and running with relatively little effort.
Perhaps most importantly the Personality Forge is fun. If you find traditional concepts of AI rather stuffy then try the Forge. Bots range from cat-girls to gods! There is also a very enthusiastic user community.
Just because the Forge is fun doesn't mean that you can't produce very sophisticated chatbots. Two Forgebots (Brother Jerome and Bildgesmythe) came 2nd and 3rd in the 2006 Chatterbox Challenge. In 2006 Adams partnered with Hanson Robotics and the Personality Forge: Robotics Edition will provide the "brain" for Hanson's Einstein bot at the Wired NextFest in New York.
The downside of the Forge is that it is totally non-standard. You can download your bot script into a text file, however the format is non-standard as are the features. Converting to AIML would be difficult and you'd lose some features. The Forge is also a one-man show - if the Prof ever gives up then the bots on the Forge will be homeless.
So the Personality Forge probably isn't the place for academic artificial intelligence researchers. But if you're fascinated by the idea of building a chatbot and don't want to bother with all the technical stuff then it's great fun. Even if - like me - you are interested in AIML etc it's still a good place to get started. Personally I find the Forge to be a far more powerful chatbot building tool than AIML - however the non-standard nature is definitely a concern.