(expecting a lot of hate...) Microsoft Visio. It's as flexible as possible without sacrificing too much usability; you can create any sort of diagrams I can t

(expecting a lot of hate...) Microsoft Visio. It's as flexible as possible without sacrificing too much usability; you can create any sort of diagrams I can think of, from simple flowcharts to complex sequence diagrams or process illustrations. The main caveat is it's a commercial tool. I have it available at work as most of my clients (to the point there's usually a preference for it). That said, on many specific cases I use other tools, like plantUML for generating more in depth sequence diagrams, and I prefer to stick to simple PowerPoint figures if I'm doing something that is very abstract - mainly because of its broad adoption; Most people will have a PowerPoint compatible tool installed at their computer and thus readily able to collaborate with me.