I did this diagram by the request of a user. According to him, it represents a schematic of BER measurement on fibre optical system, involving QD SOA. Don't ask me :-)
In the TeXnical part, it illustrates the use of matrix to position nodes in a grid, and chains to connect them. It uses some interesting tricks, such as the possiblity to add [xshift] options to indvidual nodes to alter their position in the grid, or the fit option to create rectangles that enclose a set of concrete nodes.
For defining the shapes of some nodes, I used TeX macros, instead of the interface for defining new node shapes provided by pgf, because this one is much more complex and I don't grasped it yet.