I'll be buying an iPhone 3G S this weekend, here are some of my must have apps:

For Work

Things $12.99
Things is a beautifully focused and amazingly intuitive task manager for the iPhone, There is also a desktop version of Things and the iPhone app can sync with it. Cultured Code

Billings (Not Released Yet)
Billings allows you to Track time & expenses and then wirelessly sync with the desktop version (which I currently use) so this will be a must. Marketcircle

EverNote Free
A universal capture application is only as good as its ability to catch information no matter where you are and what you're doing. With support for accessing and adding notes from your cellphone, through any web browser, or through the desktop version, Evernote is perhaps the closest option to a true universal capture tool available next to plain old pen and paper. Evernote

Connectivity

iiQuota / iiQuotaLite $3.99 / Free
iiQuota allows you to easily monitor your Phone/ISP usage directly on your iPhone. You can monitor yours, the kids or even grandma's and avoid paying excess charges. SouthFreo Software

BeeJive IM $12.99
A multi-protocol instant messaging client with 'push' support. Beejive

Skype Free
VoIP for your iPhone. Skype

TwitterFon / TwitterFon Pro Free / $5.99
I have to have a Twitter client, thanks to Simon from Em Space, I was going to buy Tweetie, but I'll try TwitterFon first. Naan Studio

Google Mobile App Free
Easy access to Google Mobile products and lets you quickly search your device or the web for whatever info you need. You can get Google Search results just like on a computer by either entering a query and tapping 'Search' or tapping the microphone and speaking your query. Google

Misc

Remote Free
Control your computers iTunes with your iPhone, also controls Apple TV if you have one. Apple Inc.

ShapeWriter Free
this little app allows you to circumvent hunt-and-peck typing when you're composing an e-mail message. WritingPad lets you drag your finger along a virtual keyboard to spell out words. It seems odd at first, but works surprisingly well once you get going. ShapeWriter Inc.

Fun Stuff (the not so essential stuff)

Ocarina $1.99
Hold the phone up to your lips and blow; four "holes" appear on your touch screen allowing you to imagine you are Link. Smule

Comic Touch $3.99
Immediately add a comic element to the photo you have just taken with your iPhone camera or from your photo library. Plasq

Pinball Dreams / Pinball Dreams Lite $4.99 / Free
One of my favorite pinball games ever, ah I miss my Amiga. Cowboy Rodeo

Guru Meditation $0.99
More Amiga nostalgia. Ian Bogost

Hero of Sparta $0.99
I'll skip the obligatory 300 reference, it's like God of War for your iPhone.. Gameloft

Music IQ Free (limited time)
It plays from your music library and you have to guess the name of the song. Easy if you have only a couple of songs, difficult if you got masses of songs and only hear them from time to time. Phase2