I’m starting to see "Bookmarklets" like this all over the place (now that I’m looking). They seem to be just links full of Javascript that the user bookmarks e.g. try me. Using DHTML, they can do some pretty advanced stuff and are easy as eat’n pancakes to code.
Note: it looks like IE has an URL length limit of 508 characters so this code won’t run in that browser; however, it would be easy enough to host the DHTML somewhere and hit it using some Ajax 
The Man in Blue > Experiments > JSCalc
JSCalc is a bookmarklet that creates a calculator on top of any web page that you are currently viewing. Just activate the bookmarklet and start typing in the figures that you need to calculate – add up prices, figure out percentages, determine the 24th decimal of Pi; anything you can count.
Right-click here and "Bookmark" JSCalc to access it anywhere, or click the link to try it out.