I've picked on a few UK usability companies who have published case studies of what they've done for their clients. These are nice, succinct stories of what can be achieved when adopting a user-focused approach for your website development or enhancement project.
The range of companies in the case studies is quite diverse: large and small, public and private.
They all had some common ground though - they wanted to demonstrably improve their services for their key audiences, and recognised that working with usability specialists would help them achieve this.
Uservision case studies
Userfocus case studies
Webcredible case studies