MSN Messenger and Yahoo talk to each other.
We've had this a while in our Live Communications Server product. The ability for corporate users to have contacts that are external to their company. We call it PIC (Public Internet Connectivity), and here's some information on it if you'd like to try it out within your organisation, and some issues you may run into..
But now, we've announced that we've entered into an agreement with Yahoo, so that if you're using Yahoo messenger, you can talk to your buddies who use MSN Messenger, or Windows Live messenger as it's now called. Pete, my favourite Web 2.0 blogger, gives you some more information on other products in the queue, and Kiltak's blog gives you the download links...
I just need to find some Yahoo buddies to talk to now...