We already spotlighted a few big apps that have made the jump to iOS 4 (and I'm sure we'll have more coming up soon), but Apple has gone ahead and put a few of its favorites on a page in iTunes for you to browse through as well. There are some excellent games on the list (PopCap didn't waste any time, and Ngmoco has a few, along with both Firemint titles). Art app Brushes and TUAW favorite Dropbox are there too, along with a bevy of other popular titles. This list definitely isn't definitive (iBooks is obviously ready for iOS 4, though it's not on there), but if you want to see what the Retina Display is like or check out multitasking on an app like Pandora, you can do so.
One interesting note on what's not on there: Apple hasn't yet updated its official Remote app, and Texas Hold 'Em hasn't been updated since it first came out back on September 1st, 2008. So even Apple isn't worrying about compatibility with all of its apps -- if you have a favorite app that's not updated often, you might be waiting a while to get full functionality with the new OS.