It is fun making tools like that, but I always find making it more fun than using it. There is nothing more boring than an overpowered character.
As to developers making what. BIS made Planescape:Torment also, not everybody likes it but for me it ranks still at first place.
As far as I know, Reflexive made ZAX which was the first 3d engine made in-house.
If you hack Lionheart’s demo, search for Zax and you shall find it inside the code.
Reflexive developed Lionheart, but Interplay (BIS) published it. (recap).
The same goes for one of Baldur’s Gate productions and not all of them.
You see, games are like movies, where there is a producer and a director, etcetera.
I am sure that Baldur’s Gate was not all developed by one entity, but I do not know if it all uses the same engine or not, yet most probably not.
Kindest regards.
[color:"yellow"]
Edited for correction [/color]
