When I get specs from my system admin, if I think it's going to take anywhere's close to a week I'll do up a UML diagram. Only takes a few hours to get a working and logical diagram. Not only does it provide me with a great reference but it's invaluable as a reference for those who aren't immediatly within the development process. That could be someone who drops into the project 2 days in or who does some tinkering with it years later.