SurgeWorks just released this free Facebook GUI PSD. The kit was is under creative commons so it can be modified as you wish without any restrictions. I always like these kits (iPhone kit released not long ago) because you can see how other PSD’s are compiled.
The kit has everything from the site: logo, UI elements, main window with the header, menu, chat window, modal components, comment boxes, buttons, message boxes, tabs, the whole works. The components are even built using vectors and blending options, so that scaling and editing the objects will not be a problem.