I love "Star Walk." Took it out into the desert a few weeks ago on a stargazing trip. Just point it at the sky and it will tell you what constellations you are looking at, what planets, what satellites... you name it, it knows it. Very, very cool! One of my favorite apps. I think it's about $5.
I also love 53 Paper, which is a fun little art / sketching app. I have no true artistic ability, but I love doodling on it. I bought a stylus (Adonit Classic, which is amazing) expressly for mindless drawing. Its free, but only comes with one free brush. The other ones cost extra. Think it's $7 total.
The geek in me loves the Marvel Comics app. I spend too much money on digital comics now, but they take up less space than print editions. Free, but comics will cost you on a per issue basis.
NPR makes a nice app. You can livestream tons of different NPR stations, or pick individual programs to listen to. I mostly use it for listening to "Wait Wait Don't Tell Me." Free.
Atomic Web is a nice web browser and, unlike Safari, it has an adblocker. I like that. $0.99
For horse stuff, I have the USEF EquiTests app. Both Dressage and Eventing editions. I think they're $10 each.
I am a news junkie. I have the LA Times, BBC News, CNN, and Al Jazeera apps. All awesome. All free.
I recently discovered Flipboard. It allows you to consolidate news, social media, blogs, etc all into one big app. You can even fold blogs in that aren't listed (I have Eventing Nation connected up, for example.) It's really freakin' cool! Looks great on iPad, too. Free.
Wikipanion and IMDB also get a lot of use for me. Both free.
Plus, I have iPhoto and iMovie. Both are useful, but I don't end up using them as much as I thought I would. The desktop version of iMovie is far more powerful and iPhoto is kind of just a novelty. I'd rather upload my photos to the laptop and edit them in Photoshop. In retrospect, I probably wouldn't have bothered buying them had I known how meh they were. $5 each.