The Best Creative Assets
The Hungry JPeg
One of my favorite sites for about everything you need for graphic content and they have the best commercial license in the business.
Design Cuts
I use Design Cuts all the time for imagery and they have a very generous commerical license.
Creator's Couture
Jessica is a designer who shares her talent with us. For using in Adobe products primarily. Her site is filled with tutorials and examples
* I am an Affiliate for some of these companies and may receive a small commission if you purchase from these links. There is no cost to you.