Jon Udell wrote an entry called The architecture of intermediation on how he as a user would like a way of adding features to a web application.
Simon Willison followed up with this article on using Greasemonkey as a lightweight intermediary
He's using Greasemonkey as a tool for annotating webpages and storing the data on a central system.
I've been thinking about something similar for a while: I read/write some German, French, and bits and pieces of Dutch, Italian and Vietnamese. The problem is that I have far too little time to spend on studying these languages, and it's far too tedious to read extensive texts in them.
Looking up words in a dictionary all the time is too tedious too. What I'd like is a way for me to a) annotate a page with notes on words/grammar etc - Simon's article has lots of great pointers, and b) automatically pull down a list of dictionary definitions for words I haven't indicated I know well enough, giving me easy access to definitions.
I think a tool like that would let me spend a lot more time reading these languages...