A long, long time ago I created an IM buddy called RPNBuddy that implements a reverse polish notation calculator as a chat bot. It ran for a while on one of my machines but didn't get a whole lot of use.
A few months ago, Hans Nordhaug, an associate professor at Molde University College in Norway, wrote to ask what had happened to RPNBuddy. I offered him the source code under the General Public License and he readily accepted, improved it and has now relaunched the service as RPNChat.
Connect to the RPNChat buddy on AIM and you can use it as a calculator. Here's a session of me calculating Roger Bannister's average speed in mph when he ran the under 4 minute mile in 1954 (3 min 59.4 s).