I was searching for a Photoshop mockup the other day and found FreebiesBug, a site that rounds up the best free resources for web and print designers. There are lots of junky sites out there, but this is one of the good ones. Get fonts, seamless patterns, buttons, icons, CSS snippets, and PSD mockups of screens to which you can add your own graphics.
Maybe you've designed a nice desktop wallpaper and wish you had a computer image to show it on. Or you need a new background pattern for your blog or Twitter, or you're looking for an interesting display typeface. Check out these resources.