Maps: the basic map app that comes with the iPhone is incredible. Sometimes it is better than Around Me for finding places, which you can then get directions to (on foot, via mass transit or via car — you can choose), and it also gives you real time traffic reports that are incredibly accurate. Has saved me many wasted hours sitting in a traffic jam!

Hootsuite and Facebook apps: awesome for staying connected as an entrepreneur who uses social media heavily for my biz.

Fandango: great for finding movie times, reviews and buying tickets so you don't have to wait in line at the theater.

Shazam: I adore this app — when you hear a song you love and don't know the name or artist, just let Shazam “hear” the song and it tells you the song, artist, album and gives you options on how to buy or share it! I'm creating my own playlist from it!

Open Table and Urban Spoon both are fantastic (as someone already said).

Epicurious is also great for finding recipes and downloading shopping lists while on the move.

Southwest Airlines: fantastic app for checking flight times and for checking in online (especially when you get your boarding pass on a first-come, first-served basis with SW). I think other airlines are coming out with similar apps. Huge time saver.

Mint.com: incredible app for budgeting and tracking your budget, and your bank and account balances. It compiles all the info into one place, and even sends you alerts for spending, balances or bills that are due. I use the online version, but the app makes it easy to always be on top of your finances no matter where you are. I must admit that the B of A app isn't bad either.

TaxTracker: you can track expenses and sync with Quickbooks — you can even take photos of your receipts and save a pdf of the receipt for your records (much easier than the paper records).

iTunes and iPod: they also come with the iPhone, but they are great for finding podcasts and listening to them on the run. Also, who can live without great music? With the iTunes app, you can download music immediately without having to sync to your computer.

Three games that my 6 year old loves (great for buying yourself time for an adult conversation or getting a little work done!):
Littlest Pet Shop
Horses (is really a quiz on different horse breeds, which for us, is hugely entertaining)
RhinoBall (from the movie Bolt)
My daughter loves all three (and they aren't violent or noise dependent, so you can silence the sound if you need to without ruining the experience for your child).

There are also great educational apps out there like:
Math Magic
Sums Stacker
bunch of spelling or ABC apps for younger kids