This week I've been on a course on Axure - a piece of software I've played with in the past but not used in earnest.
I was amazed at how quickly I could knock up prototypes for a range of search interfaces we've been comparing.
I've been using Visio for a good while now, but am definitely an Axure convert.
Axure - "...for rapidly creating wireframes, prototypes and specifications for applications and web sites."
AX Trainers - Axure training delivered by usability professionals