Shameless plug: I'm working on a HTML5-based high fidelity mockup tool called jMockups [1], which is intended to be an alternative to Photoshop and HTML for creating website mockups.
You can't link the mockups together yet and its still a bit buggy, but long term I want to make it effortless for even the most design-challenged developer to design beautiful websites.
Let me know what you think: matt@jmockups.com.