The one I probably use the most often (daily, if not several times a day) is Hipstamatic, a camera app. It tends to help make even boring pics look good, so I love it.
Yes, the sunsets in FL are gorgeous just about every night, but "normal" photos don't come out looking like this:
Others I use quite a bit are the WDW Times Guide (takes the place of the paper Times Guides you can get at the front of every park)...since I'm local now, I use it constantly to plan which park to swing over to for a couple of hours at night after work, etc. The WDW Dining app contains a list of every restaurant on property, broken down by location (including resort restaurants and DTD), and lists complete menus, pricing, and dress codes (if applicable) for each place. You can mark specific restaurants as favorites and access them quickly. It's really a treasure trove of info!
I also like Photoshop Mobile, which is good for quick photo editing on the go (cropping and rotating, simple stuff like that). The Walgreens, Target, Publix, and
Best Buy apps are all nice for both finding local stores (and their hours) and checking out the weekly ads.
Toy Story Midway Mania is remarkably similar to the ride (and Wii game) and is ridiculously addictive. It costs something like $4.99, maybe, but is totally worth it, IMO.
If you have DirecTV, you can use their free app to program your DVR from anywhere, which is AWESOME. There've been so many times I've been out and about and remembered something that I had wanted to record but forgot about, and I could take care of it from where I was.
If you love live music, iConcertCal is great. You let it access your location, and it can tell you who all will be playing in the area over the next several months. You can also enter different cities and check out who's going there, as well, and from within the app you can buy tickets and get directions to venues (something that came in very handy when a friend and I were bumbling around downtown Houston a year ago after driving all night from Nashville). Oh, and I forgot--it automatically scans your iTunes library and saves those artists as ones you'd be interested in, so they're given priority in the listings of upcoming shows.
WhoCalled is free, if I remember correctly, and lets you enter a number that's called you and will tell you to whom it's registered.
Shazam is also free, I believe, and will identify music that's playing around you. Like if a song comes on in the car and you know you know who sings it but can't remember and it's just KILLING you, you can launch Shazam, hold up the phone in the direction of the speaker, and it'll listen to the song and then spit back the identifying info at you. It still seems like there must be magic or witchcraft or something involved with this one that allows it to work and be accurate constantly, but I love it!

My mom always thought the hype about the iPhone was a load of hooey, but the first time I demonstrated Shazam to her, she was like, "Oh, man, that's COOL!"