Well, carmi - there are just about everything for everyone and your personal taste dictate what apps are useful and what aren't
But I'll try
NAVIGON - probably the best navigation app available. The $80 price tag is a bit of a turn off for most. But most standalone Navi's cost that much if not more and you're going to have your phone on you anyway. They constantly update and release add-ons. They are serious about making you happy. Recommended for frequent drivers. Now with the Map app's ability to share your location with others through email or SMS, meeting up with friends at a random house party is just a few clicks away- even a drunk can do it! TomTom is another option. Drive safe.
Google app - There are talks of Apple switching its default search in Safari to Bing. Only Apple fanboys and Goo fanboys care about it and to the rest of the users, this app with its voice activated search is more than enough.
RedLaser - if you often do market research on Google before you buy things, this app is a cool concept. It scans the barcode and searches the prices through Google. People claimed to have saved money using this. Similar apps are available in the store.
Air Sharing - Turn your iPhone into a wireless thumb drive. Similar apps are available.
ChemTouch - Periodic Table. A lady at a grocery store was doing her crossword puzzle, she notices that I'm Asian so she asks me "what element is Tc?" so I replied "I'm not sure, but give me a second and I'll look it up in my iPhone." "Technetium!" "You're awesome!" "I know". True Story.
Banner - Customizable scrolling message. I have FUCK U on stand-by for road rage incidents. Similar apps are available.
myLite - Turn you iPhone into a flashlight. Color and effects are available for your next acid trip session. Similar apps are available. I'll mention this even though I've learned that opening up the SMS app or a blank white Safari page is good enough. Get it if it's free. This might save your pinky toe from damage during the next power outage.
TED - Ideas worth spreading. Great for while on the shitter.
Shazam - Ever hear a song on a YouTube video and can't find the artist name? Open Shazam and point the iPhone's microphone to the speakers. I've had about 60% success rate. Purchased 4 albums so far thanks to Shazam.
There are all kinds of creativity apps for music and graphics. Worth looking into.
Same with Photography. Phote editing, filters, manipulation, uploading to a service like Flickr. They are all in the Photography section.
I'm not too keen on using a phone for serious photography. I say it's not megapixel, it's the optical glass used for the lens. But no problem for casual Facebook photos. /rant
News apps offered by your local stations might be a good way to be in touch with your city's news, traffic, sports, etc. They offer things that apps like CNN and USAToday can't offer.
Yesterday, I looked outside and I saw a bunch of white crap growing on my yard "omega wtf is that?!" I said. Later, I pop opened the KARE 11 app and lo and behold, they're talking about the outbreak of snow mold. Isn't the information age great?
There are great deal of shopping apps available. I use Amazon, NewEgg, eBay, Overstock, Target and Best Buy. I don't shop on my phone that often, but it's useful to have access to your order status and things like that while you're away from the desktop.
Games, of course the games. This sector was a surprise even for Apple I think. Lots of games on the iPhone. Games are a good way to drain your batter in a short time, but if you turn off the sound and vibration and lower the brightness, you can get a decent amount of gaming done. Some apps that get replay are;
HoldEm - poker
Scarbble - is Wu are word?
FlightControl - simply addictive
Tiger Woods - he will be back!
Madden NFL - totally unrealistic, because Adrain Peterson never fumbles in it, but it's fun.
Battleship - play with friends. what more can you ask for?
Do most you app downloading in iTunes. Faster and easier imo.