A few years ago Jared Spool told the story of a project he worked on in which a relatively simple alteration had an incredibly profitable pay off. He's recently written a new article telling the back story.
If you've not come across this story before, it's well worth a read. Jared's company do a usability study for an e-commerce company. The designers replace a mandatory registration process with an alternative purchase step and sales increase by $300 million in the first year.
A fantastic story, and really inspiring for anyone trying to embed some user centred research into their website management processes.
The $300 Million Button - article by Jared Spool for uie.com
The Back Story for the $300 Million Button - blog post by Jared Spool