Here's a cool resource that I've been evaluating: The LiveSurface Image Template Library. These are not just stock photos. They're pre-masked, multi-layered high-res photos of billboards, stationary, packaging, vehicles, etc. that contain built-in 3D surfaces. What this means is that you can use the photos to quickly visualize what your designs will look like in context, create high-quality client presentations, or create final high-res images for use in print or Web marketing materials.
The images take advantage of the vanishing point feature of Photoshop CS2-CS4.
The layers and masks are well-labeled and well-organized, and instructions are included.
It took me just a matter of minutes to create the images below.