Here you can find animated graphics from alphabets to transport. Just go down the alphabet to find the animation topic of your choice. Probably the best organized of all the sites. Animated GIFs page - There are both cool sounding wavs and animated gifs here. This site has all the clipart you could want, and it's organized by category.
Site also includes alphabets, fonts, backgrounds, borders, and backgrounds.
Not all of these sites are free. This site has a bunch of free art categorized by theme.