UML, class heirarhies, documentation: looking for app

[caibbor]

I am coding a game engine that is totaling over 19 thousand lines (not including blank lines and comments). I am currently using Dia to graph out class inheritance, and it works fine for that but I'd like to be able to zoom into each class and explain how it functions in relation to other classes. this is hard with Dia, if not only for the fact that I'd have to manually edit each class name if I change it (every now and again I do a global search and replace to make things more intelligible). is there any free/foss application out there that can helpe me document this better? code comments and inheritance graphs only go so far.