In your desktop browser, YouTube allows the modern equivalent of channel surfing: browsing related videos while you watch a clip. Unfortunately, the YouTube app for iPhone doesn't match that enviable capability.
Enter Youtopia, which gives you a more complete (and entertaining) YouTube experience by letting you "channel surf" while the current video plays on.
With a tap of the screen, the app toggles between two modes: full-screen video viewing and an overlaid interface. The latter displays various options (Featured, Channels, Search, and so on) in a toolbar down the left side and a scrolling thumbnail list of related vids on the right. Playback controls appear across the bottom.
It's a simple tweak, but also a cool one: now you can easily flick through other clips you might want to watch, then tap one to immediately start watching it. And if you sign in to your YouTube account and load, say, a subscription channel or favorites list, you'll see thumbnails from that selection (as opposed to randomly selected clips).
Another nice perk: when you search or browse for videos, Youtopia displays user ratings; Google's YouTube app does not. There's also a handy Data Plan Saving Mode that drops videos to a lower resolution when your iPhone isn't connected to Wi-Fi.
My only complaint is that Youtopia doesn't shrink the currently playing video while you're browsing, meaning you lose a little more than half the image when you toggle away from full-screen.
Even so, for anyone who loves channel-surfing YouTube on the go, Youtopia is definitely worth a look. And you can't beat the price: it's free.