iStockphoto is one of my favorite sites around. They have a ton of great stock photographs, illustrations and videos for all your design needs. Their prices are incredibly cheap and many professional artists submit to iStock, so the selection of work available is amazing.
Not only will iStock make your life easier as a designer by giving you access to tons of great high and low resolution stock images and illustrations, but they also have many other features on their site. You can create your own profile and network with photographers, illustrators and designers easily. They also have a free image every week, an artist of the week and a design of the week.
Istockphoto also has a great forums. The design section has many helpful members that are very generous and willing to share information and tips.
Its free to sign up and you can buy credits to download stock photography, illustrations and videos. www.istockphoto.com