16 x 16 favicons (the little icon some websites have, that can be seen in the address bar or on tabs if your browser uses them)
32 x 32 Windows XP icons (to decorate the shortcut folders on your desktop for example) or traditional sized link button for affiliates
88 x 31 traditional size for website link buttons
50 x 50 AIM icons, DeviantArt.com…
96 x 96 MSN avatars
100 x 100 LiveJournal icons, message boards…