I’m using this for my final year project, a decentralised web annotation system that lets you annotate pages, storing your annotations locally and then sharing your public annotations as a feed (similar to the way RSS aggregators work). The trick there is to run a local web server on some port, then have the Greasemonkey user script (eventually a full extension) communicate with that local server to store and retrieve data. I’m using Ruby on Rails’ built in WEBrick server to prototype the service, and it’s working a treat.
As Sam Ruby said: I want. I want. IWantIWantIWant.