Platypus works in conjunction with Greasemonkey to let you edit web pages and then save your changes as a Greasemonkey script so the next time you visit the site, it will show the site as you designed it last time.
Platypus requires Greasemonkey 0.3.3 or higher to work, but that doesn't seem to be a problem with most extensions.
After working with the extension for a while, I've found an additional use for the Platypus. This extension may be used in editing an existing web page that a client likes, but wants certains visual components eliminated from the screen ("What do you like about the page?", "Which features do you want to remove?", "Could you move that component above the heading?")
One key feature in Platypus is the color-coded display for "view source." However, if you have Jennifer Madden's View Rendered Source, Platypus will easily detect the extension and use that instead of the view source that platypus supplies.
I haven't run into any problems with the extension and its a utility I use once in a while. If you are modifying existing web pages visually in front of a client or doodling to find the best layout with your current site, Platypus provides a top-notch layout extension for designers and developers alike.