StumbleUpon's new iPhone app is a killer way to waste time
There's a toolbar for quickly skipping to the next random site or rating with the thumbs up/thumbs down buttons, plus thumbnails so you don't have to load a whole page if you're not interested. Logging in tailors the stumbing experience to your interests and allows you to sort by the topics you've listed in your profile. Unfortunately, you can't edit your profile or interact with your StumbleUpon contacts using the app, but you can browse your friends' favorites lists.
I do have one small gripe with StumbleUpon on the iPhone, though: I wish there were some way to filter out sites with extensive Flash content. StumbleUpon handles YouTube videos admirably, but having to skip past Flash sites on an iPhone is a little annoying. Fortunately, you're always just one tap away from the next random site!