Web service/infrastructure: HTTP Observer
A generalized service for polling a HTTP resource so you don’t have to.
The Observer Server has a list of URLs to hit, how frequently to hit them, and who to tell once they’ve changed. Observers register with the server and maintain a persistent TCP connection (to avoid NAT issues; can possibly be changed once IPv6 actually happens).
Example ideas: FOAFster (distributed Friendster/MySpace/Facebook using FOAFs); RSS reader; Campfire implementation; a 4chan plugin for Firefox.
This can be made easier with libraries for various implementations; a libobserver for C; an observer.js; a Ruby gem probably called something irrelevant like Golden Hawk; pyObserver for Python; and so on.