I've heard of library helpdesks having Instant Messaging accounts to allow easy access to support, and thought this interesting because it's more immediate than email.
Lindsey blogged about Plugoo which is a service that allows people to communicate with your IM account direct from a web page. I've created an example that links to my personal account below.
I like this because it lets users know if you are available and allows you to chat with them, without any barriers to them at all (e.g. logging on to email). On the other side it is impossible to identify people so you couldn't accept requests to change account details through this, and while you were logged on to IM you'd potentially be interupted all the time.
Even so it is an interesting use of IM technology.