This whole AppStore ecosystem is ingenius. The prices for most of the apps are low enough that people will buy more on instinct instead of worrying so much about a $35 application. Here are some of my favorites:

OmniFocus
It's a little expensive, and the sync process is a bit slow, and it could really, really use a quick-entry companion app, but still it's really nice have access to my GTD system in my pocket. I used some web-based stuff before, but when I'd get into the grocery store with a weak signal, it rendered those useless.

You can view your stuff by Project: (here's my Projects : Shopping List : Wine)

OmniFocus - Project

And here's Context : Errands, where I have several options to choose from. Grocery store has subcontexts like Produce, Dairy, etc.

OmniFocus - Context

And then the really cool context viewer, the Nearby button. You can set up the contexts to look for a specific store, kind of store, and show you ways you can accomplish things nearby. Like for my wine list, I have it set to search for wine stores, so it looks to see if there are any close to my current location. You can also set it as a particular place, so that it knows where home is, where work is, etc.

OmniFocus - Nearby

WeatherBug

Nice little freebie for checking the current weather, including a Google map interface with weather overlaid on to it. And you can pinch zoom in and out on the radar. All it needs to be completly awesome is a radar in motion.

WeatherBug

Twinkle

Location-aware Twitter client. See what people are talking about within a 25, 50, 100 mile radius of your current location.

Twinkle

Toy Bot Diaries

There's a rumored update for Super Monkey Ball coming. It's been the game most people have bought. It's neat, but hard. The update may fix that. But that's a game created for another platform that was ported to the phone. Toy Bot Diaries was written for the phone and it feels like it. Great little puzzler/platformer at a great price.

ToyBot Diaries

Motion X Poker
One of those games you can play when you have 2 minutes in line. Great physics and a great looking game.

MotionX Poker