A new to me website, but obviously not to 35 thousand other people… Gliffy is a web-2.o cross between writely and MS Visio. Lots of drag-n-drop tools for work flows, wireframes, floor plans. Plus, it's a web application that requires no additional software.
The other cool part is the ability to collaborate with others. As an example, here's a 30 second diagram of my living room. I could add my wife as an editor to that file. Then, she could rearrange everything before getting me to schlep our stuff 6-ways from monday! ;-) There are probably some nifty possibilities for Libraries & Law Firms too.
Visio killer? maybe. It was certainly impressive on the first run.