Openclipart.Org is a popular service that provides you with high quality clip art images and icons which you are free to use in your website or anyhwere, all images are in public domain so you are free to modify or built upon those images by downloading them to your computer.
Simply visit its web address and start browsing the images. The service also allows you to upload your own clip art images and if you are a developer then you can make applications using its API key.