I was reading a news article from WordPress about the usability test done with WordPress 2.5 and I was impressed with the detail of the report.
If you are, just like me, an applications developer, you will find this usefull even if the application in the report is not yours. You will see common usuability mistakes and most important the point of view of the users.
Check it out. It is a very interesting read: http://wordpress.org/development/2008/10/usability-testing-report-25-and-crazyhorse/
P.S. Check out the PDF on the end of the article ( is far more interesting 
)
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