Product Planner is a slick and simple web application that allows a user to "discover, create, and share user flows."

Creating a user flow in Product Planner provides you with the ability upload images - screenshots from web sites or wireframes, for example. You can easily add steps to the flow and arrange your process accordingly. Give it a shot! I can see this being quite useful for creating quick user flow mock-ups heading into a brainstorming or planning meeting for a site project.

This is an example of a existing flow in the gallery - the Twitter Sign-up Loop.

(via Squared Eye)

UPDATE: After some further tinkering, it appears as though the application only provides 3 flow orientations - horizontal, veritical, or loop. This could prove to be fairly limiting for more complex flow diagrams.