Was just referred a service online which generates sequence diagrams for modeling communication between components. You type in the interactions in text and it generates the image for you on the fly!
This is a great find and will try it out in my next project.
Edit: And if quick sequence diagrams didn’t do it for you, here’s a program that makes almost every other type of diagram Visio-style. There is a one-diagram limit if you don’t want to pony up the monthly cash, but it’s great for one-off diagrams!