I end up doing a lot of software infrastructure architecture diagrams. The product I work on has a lot of components and interacts with a lot of things, and the people we sell it to always want that mapped out. Frustratingly, they don't ever seem to be able to comprehend the generic diagram, and require it to be remapped into their actual infrastructure, which I never have any idea about, and am usually at least two Kevin Bacon's away from any kind of access to.

Otherwise I do a lot of off-the-cuff statemachine and sequence diagrams, I guess you would call them. Very often literally scrawled on a napkin and then photographed with my phone.