Jeff Sauro pulls together a collection of articles, software and other useful stuff relating to planning website and application structure and navigation.
I've been using Optimal Sort's online card sorting tool recently and had a really good experience. Actually convinced me to do a bit more card sorting after being left jaded from data analysis hell on analogue card sorts a few years back. I'll write about my experiences in the near future.
In the meantime, some great stuff here:
20 Resources For Evaluating Website Navigation - blog post by Jeff Sauro on measuring usability.com