iTunes obviously has a great deal of data about its sales; Apple is quite aware of the top sellers among its music, apps, movies and podcasts. That information is also available through RSS feeds. If you want to share that data on your website, or even if you just want it in your RSS reader, you can use the official iTunes RSS Generator to get the right feed.
You can refine the RSS feed created by Country, Media Type, Feed Type, Size and whether to allow explicit content. The Media Types available include Music, Podcasts, Audiobooks, TV Shows, Music Videos, Movies, Mac Apps and Apps. There are obviously many options under each media type, so you should check it out and see what's available. Just punch in the criteria you want to follow, hit "Generate" and then hook up that feed to an RSS reader, and you'll be kept up to date on the latest changes in the iTunes stores.