iStockPhoto is a great site for all your image and video needs.  They have a vast library of high quality images and videos. You buy credits and then spend them on the content you want, one download at a time. If you just want a small image for a website you can download it for $1, but if you're doing a full scale video production and need some High Definition video you can get that too.

Best still is the fact that you can upload your own content, and if it's good enough it will be included in the library for others to use and then you can start making money too!