If you enjoy hands on, in-person usability testing and you have clients who don't need to see ultra-slick deliverables, paper prototyping can be a great tool to elicit user responses and gain insights as you evolve an interface design. The Paper Prototyping DVD from Nielsen Norman is a eye-opening introduction to the possibilities.
You can have test subjects work with low or high fidelity wireframes, hand-sketched or fully designed in Photoshop . The advantage of working with paper is that it's fast, flexible, inexpensive and fun. And, it encourages conversations with users in ways that interactive prototypes and real websites don't.
For instance, to test content, you can lay a cardboard window on top of the page to indicate a scrollable viewing area. When the user needs to scroll, he waits while the test facilitator moves the window down the page. While this isn't the way people really interact with content, it slows down interactions and provides opportunities for users to share their thoughts.
There are several other user testing scenarios covered on the DVD, but what I enjoyed most was the section that covers all of the different, crafty 3-D tricks with construction paper and post it notes to add features like pop-up boxes, drop-down lists, and other controls to your prototypes.