UI-patterns.com: Lazy Registration
A List Apart: Articles: Testing Accordian Forms
Accordion forms use dynamic interactions on a single web page to hide and reveal sections of related questions as people go through the process of completing a form
Effective A/B Testing
Google Website Optimiser
Website Optimizer, Google’s free website testing and optimization tool, allows you to increase the value of your existing websites and traffic without spending a cent. Using Website Optimizer to test and optimize site content and design, you can quickly and easily increase revenue and ROI whether you’re new to marketing or an expert.
The Anatomy of a Perfect Landing Page - Formstack