BuddySpace is an instant messenger with four novel twists: (1) it allows optional maps for geographical & office-plan visualizations in addition to standard ‘buddy lists’; (2) it is built on open source Jabber, which makes it interoperable with ICQ, MSN, Yahoo and others; (3) it is implemented in Java, so it is cross-platform; (4) it is built by a UK research lab, so it is 100% free with full sources readily available.
Also available for Mac OS X.