Surely You Jest
The Blue Obelisk maintains a Wiki page discussing some of the uses of Greasemonkey scripts in chemistry. Greasemonkey is a wonderful tool for augmenting existing Websites, and these scripts do some remarkable things, but they don't fall into the category of cross-browser, installation-free, general-purpose tools.
An early compilation of resources can be found here, although all of the listed sites have since either disappeared or converted to other content.
What Would Be Most Valuable?
We might begin by asking: what makes chemical information difficult to work with in the greatest number of cases?
Since the earliest days of chemistry and computers, it has been clear that one of the distinguishing characteristics of chemical information is the central role played by chemical structures and the difficulty in accurately representing and processing them on machines.
Would the performance and usability of such toolkits be high enough to make them a serious choice for use in chemistry-driven Web applications? What would the availability of such a toolkit enable that is currently difficult or impossible?