[Bonzabonz]

I have used CadSoft EAGLE for several years now to do all sorts of schematic diagrams and basic board layout. They have a freeware version that is pretty good. It can be a little tricky to learn at first, but I was able to pick up on the basics and make good schematics after an hour or two of online tutorials and youtube videos.

If you're looking to do more block/wiring diagram stuff (similar to the control system diagram that was mentioned earlier) I would second the suggestion of powerpoint; I have seen some very professional and detailed diagrams produced using powerpoint.