One of the greatest features of Gliffy is the ability to share your drawings because it's an internet-based application, no matter your operating system, as long as you have (Macromedia, now Adobe) Flash, installed, which 95% of internet users already do. Even CNET was impressed, and the functionality has improved since their review was written last year.
It looks--and works--a lot like Visio, which is a drag-and-drop Microsoft product that you must purchase for each program user.
Gliffy is free, and files are stored on their server. It doesn't appear to offer the preciseness Visio can offer a professional builder/designer relating to issues of drawing in scale, but in many situations, that level of detail isn't necessary. It also doesn't offer the automatic updates to the version you're working on like Google applications, but the sharing is still convenient.
We could see drawing out a floor plan and using it to share options for moving things around with our friends, who in turn can play with the layout and offer other options to us, all from wherever we are in the country because of the sharing ability of this program. Now, if they could just make the name a little less cheesy...
Their site links to a tutorial/demo for more information.