Does anyone know.....

#1 Try searching for muds or mushes.

Try looking at activeworlds (www.activeworlds.com) and exploring what you can do with bots.

There are a few free everquest clones out there; try one of them.

Go to freshmeat (www.freshmeat.net) and search for rpg.

Consider buying Torque (see the front page of this site). Torque will involve a lot of work, however.

But first of all, come up with a design of what you want to do. I think you might be approaching this with too vague of an idea of what you want to do.

Gamasutra might have some interesting articles for you (www.gamasutra.com)