Lost Garden: Danc's Miraculously Flexible Game Prototyping Tiles
SpriteLib - SpriteLib is a collection of static and animated graphic objects (also commonly known as sprites). It was created to provide hobbyist game developers with an assortment of images to use in their creations. Because of SpriteLib, developers don't have to waste precious time or money creating graphics from scratch.
The Krush Mobile App is a good example of excellent use of gradients and a possible alternate execution of the hamburger menu for iOS.
Patterns in Interaction Design
Free game graphics - Graphics for small worlds
SCVNGR's Secret Game Mechanics Playdeck
Some companies keep a playbook of product tips, tricks and trade secrets. Zynga has an internal playbook, for instance, that is a collection of "concepts,..
openGameArt is a collection of game art related resources under multiple flexible licenses
Mobile Game Patterns and screenshots of what different games are doing in their various stages.