Systems Outline | Project planning and misc | Forum

According to the current version of the Engineering Process (which wasn't exactly written collaboratively, I realise, but nobody has complained about it yet), with OHKLA we should now be doing the Systems Outline Phase:

"This involves the identification of all required systems and subsystems
at a moderate level of detail (e.g. power system, communications
system, navigation system, etc.), the minimum requirements that each
system must necessarily meet, and the required interactions between
each of the systems. The ultimate output of this phase should be a
number of block-level functional diagrams related to the project."

This should not be particularly hard or take particularly long for OHKLA, but there's been no discussion so far on how to actually do it.  Does anyone know any good free software for producing nice block diagrams or will one of us just have to do it in Gimp or the like?  With regard to listing requirements and interactions, should we just do this using text in the Wiki?