Vector images are great for designers, mostly due to their extreme flexibility. With no fixed resolution, they scale up and down in size with no loss of image quality and work well in print, flash, backgrounds and more.
There are hundreds of sites that offer free vectors. Some are completely devoted to vectors and vectors only, while a few have them tucked into a freebies category. The point is, there are plenty of free vectors out there if you know where to find them.