Cacoo is a free, drag-and-drop, in-browser chart-making tool that lets you whip up a flowchart in a hurry. It’s not the best-looking app, but it’s pretty foolproof, and you can have multiple users editing a chart simultaneously. (Kind of like a more chunky Google Wave? RIP)
I can’t imagine that anyone who reads HOW Interactive Design would use Cacoo to build wireframes, but it could be super useful for creating simple workflow charts or diagrams for the office.
In any case, Cacoo could be a real boon for using with a client who wants to nitpick or a boss who wants to instantly revise your work (but who doesn’t have Illustrator or Photoshop).