For a long time I've wanted one of these, to serve as an "interruptility" indicator to others: green = "I'm looking at the internet, please interrupt me so I can possibly do something more useful", amber = "I'm working but okay", red = "only interrupt me if it's truly urgent". It could be programmable, so that (for example) it could automatically go to green when Firefox had HN as its frontmost window.

Imagine a lab or office in which everyone used an LED in this way. It might be nice.