I was asked to provide some UCD case studies at work, preferrably those from global companies. My colleagues found these interesting:
The Cost of Frustration
http://www.uie.com/articles/cost_of_frustration/
Gradual Engagement Boosts Twitter Sign-Ups by 29%
http://www.lukew.com/ff/entry.asp?1128
Case Study: Fewer Input Fields Increases Conversions
http://www.lukew.com/ff/entry.asp?910
Google
Recruiting user participants for testing - http://blogs.zdnet.com/micro-markets/?p=472
Facebook
Gathering user feedback before implementing a new design change - http://user-vision.blogspot.com/2008/07/new-facebook-usability-research-on.html
Amazon
Letting their users decide - http://www.uie.com/articles/magicbehindamazon/
Tripadvisor
“Since joining in 2004, Petersen has overseen tremendous growth of site traffic, revenue, content and membership, including driving user involvement through the launch of rich product offerings such as maps, video, personalization and intra-member email” - http://www.tripadvisor.co.uk/PressCenter-c5-Our_Team.html
Expedia
“The usability testing course was delivered at a comfortable pace and in a group small enough to benefit all levels of experience. Thanks for a very enjoyable course!
Graham Farrugia, Expedia.co.uk”
Microsoft - Halo 3
http://www.wired.com/gaming/virtualworlds/magazine/15-09/ff_halo?currentPage=all
The button one is my favourite though :-)