Here are four tools that I've found useful for creating design mockups and comps:
3. MockupEverything lets you create easy mockups completely online, no knowledge of Photoshop needed. Just select a template and upload your art to create a mockup.
4. lorempixel is a placeholder image generator. Just enter the width and height of your desired image in pixels, choose a subject category, and indicate if you want color or grayscale. A random Creative Commons licensed image that matches your specs will display, ready for you to download and use in your design comp.