Free Full-Layered Facebook GUI PSD Kit
The idea behind this kit is to speed up the prototyping of Facebook application UIs and Facebook fan pages, sparing you from drawing all the comps and letting you customize all the text, buttons and data as you need. The kit is free to use in all projects, without any restrictions.
Facebook GUI Free PSD Resource
A Facebook graphic user interface (FBGUI) resource kit for Photoshop to make your work easier.
A sizable collection of elements for creating wireframes for Facebook applications.
Omnigraffle Twitter Widget Stencil
This set includes a selection of useful Twitter widgets and badges.