Wow, this is amazingly useful for complex PSDs.
photoshop
Photoshop Grids
Photoshop grid templates for various screen sizes and common constraints.
iOS 6 GUI PSD iPhone 5
An Explanation of Photoshop Blend Modes
350 Pixel Perfect Icons
Free for both personal and commercial use. Nice.
iOS Photoshop Actions & Workflows
A must have for all iOS designers.