Answer in the form of a hail of bullets
Grindr, Shazam, Google, i.TV (if you watch television), LiveJournal, Now Playing, and Facebook if you do so.
For Twitter, I use the free version of TwitterFon. I hear Tweetie is also good.
NY Times has a nice app. I wasn't crazy about the Facebook app. m.facebook.com is uglier, but has more functionality.
There's a program called iConcertCal that scans the MP3s on your iPhone, gets your location, and tells you who's playing soon. I live under a rock, so I find it helpful.
You don't strike me as a baseball fan, but if you are, the MLB At-Bat app is wonderful.
The iPhone default weather app is decent, but the Weather Channel's app is better.
There's no default to-do list on the iPhone. I use an expensive one called ToDo, and then I found that Toodledo has a much cheaper one. Oops.
Yelp and Urbanspoon have slick apps that tell you where good food, etc. is nearby.
If you're willing to jailbreak your phone, there are terminal programs and backgrounding apps available. There might be an emulator or two available too.
Finally, games. I'm a Reiner Knizia groupie, so I have his 3 iPhone games. The dude has designed over 100 board games, and spits about 10 more each year. And they're almost always very good. Start with Poison.
If you like tower defense games, I've dug Fieldrunners and GeoDefense.
Ancient Frog and Zen Bound are both relaxing and beautiful.
Erm, that's all I can think of off the top of my head. Have some Kool-Aid, new cultist!
OffMaps is good; it's like the Google Maps app that's built in, only it uses OpenStreetMap, and can download and cache regions of maps for offline use.
As far as games go, Edge is the best aesthetic experience I've seen in the format (though may have been pulled from the App Store thanks to a trademark troll). The Rolando games are good too, though (at least with the original), I'd recommend playing them with the sound off.
Also of note: Google Earth, Tone Pad (a Tenori-On-style music toy), iRail (international railway timetable searching, at least in Europe), and Stanza (the e-book reader).
Buddha Machine - it's the color of the new millennium!
I play solitaire...i think Solebon is the app maker, i user Sol Free.
I agree with Yelp, Urbanspoon, Facebook, Livejournal, Google...
I also have WikiTap, Fandango (for movies), Flashlight..and some other games.
For me (starting from first home screen and moving to more distant, that is less often used, ones) some notable and recommendable apps are:
- PCalc for fancy hexy calculatory goodness
- The Weather Channel - hour-by-hour forecasts for the win, though I don't know how important that is for you in The OC
- OmniFocus - because I'm task-nerdy like that
- Dictionary.com's dictionary/thesaurus
- ByLine - for viewing/syncing with Google Reader
- Gas Cubby - tracking MPGs and service schedules
- Trapster - watchin' out for the Po-Po
- Emergency Radio - like a police scanner in your phone
- Now Playing
- ~2 pages of games and diversions that I can detail if you want me to
- Mocha VNC - this is the full version, not the Lite one
- pTerm - for ssh/telnet
- Rowmote - I use this with my Mac Mini media center
- AirSharing - I bought the Pro version, but really didn't end up using more than the standard version's feature set.
- Snap - it's like a basic GUI version of nmap; network scanner, port scanner, bonjour browser, etc.
- Ambiance - good background sounds
- Electronic Reference - resistor/capacitor codes, etc.
- Mathomatic - basic algebra transforms and solves
- Public Radio - I actually don't use this. I can recommend it, but I end up getting everything I care about via podcast anyway, so don't really do the streaming thing that much.
That's pretty much everything I can recommend. I have a few PDX-specific apps and a ton of generic things like Facebook, MySpace, LinkedIn, a few GPS apps, an IRC client and whatnot, but they're either too narrowly focused to recommend or too broadly known to be worth mentioning.
get the thing that make the light saber sound when you shake the phone.
2 Across! Cheap app, practically unlimited free crosswords from major newspapers! I am a severe addict. The Wall Street Journal ones are pretty good. MUST HAVE, if you're a puzzle person.
Flashlight has already come in handy a couple of times -- turns your screen pure white. Clever.
Shazam is free and made of magic. Music-identifying software: you just hold the iPhone up in the direction of the music, and it'll be able to tell what it is 60-70% of the time.
TweetDeck is the free app I use for Twittering.
FTPOnTheGo and TouchTerm (SSH) are pretty rad, as far as networking goes.
OneTap is handy for movie-related stuff. Just don't click on the trailers, though, because there doesn't seem to be a way to stop them.
WiFiTrak is a decent sniffer.
Frotz! Fuck yeah! Text adventures are where mobile computing is at, baby.
Do not go without the free Google app thing. Very handy, integrates nicely with Safari.
UrbanSpoon is good for finding restaurants and restaurant reviews while you're out and about. I just found a really nice little Indian restaurant through it that I'd never heard of before.
Make a bookmark in Safari for Enigma's Dr. Horrible Van Remote thing.
Everything else is, for me, cool but not useful on a regular basis.