Take a look at An Introduction to Topic Maps on msdn. It's written by Kal Ahmed and Graham Moore of Networked Planet. It's more informative and more up-to-date than the introduction on my website :-(
Kal Ahmed has been heavily involved in TM4j, an open source implementation of the Topic Map API. Graham and he have now created a (commercial) TMCore topic map engine for .NET
I've been playing with Topic Maps for a while now. Topic Mapping has incredible potential as a technique for knowledge creation and dissemination, but uptake has been slow. Let's hope that interest from the Microsoft developer community will be the trigger for liftoff.