I've been doing a lot of flow diagrams and PlantUML has been invaluable. I specifically like it because sharing the flow means sharing some text, which itself is vaguely human-readable. There's an online renderer (bottom of the page in the above link), which means simple diagrams are also quick to create and view. It's got a few different modes that it understands but I'd definitely add it to the list when you need simple vector graphics.