Much has been said about the vast number of applications that have made it into the app store, unfortunately, most of it is crap. That said, there are gems in there to be had, whether they are free or worth the nominal fee, so I thought I'd start a thread for recommending good apps.

I'll make a start..
Photography wise, DOFMaster is excellent and Photoshop Mobile is really handy.
DOFMaster allows you to select your camera, your focal length and aperture and it'll give you the depth of field for a chosen focal distance - essential for those of us with a nifty fifty with its knife-edge DOF!

Batterymaster is also an excellent app, breaking down your available battery life into talk, wifi, audio, video and standby time. (59p)

On the photography front as well as PS Mobile I also use Format126. I use the official Flickr for iPhone app. mainly because there's loads of them out there and I'm never very far away from a computer connected to the 'net so anything the app. doesn't do I can do on a Mac.
I'm still waiting for the stalled Flickr application that Edd was going to do before I splurge any money on a Flickr application.
For non-photographic purposes I use Skype, Facebook and a great application called Geocaching.
This last one is a fun little thing that gets you out and about looking for items that other geocachers have placed. You don't have to involve yourself more than simply going around nearby caches, finding them (which can be a little tricky sometimes) and signing a bit of paper. After which you login to the 'site using the app. and update the cach log.
It's like a modern day treasure hunt application. (8+)

Mark

I have a pile of photographic apps - ranging from ones that make it look like you have different cameras, to ones that produce anaglyphs and stitched panoramas. Probably too many to list here. I have in my mind to do a breakdown of the apps I have after a prompting from someone on Twitter a few weeks ago.

There are a lot of crap apps, and at lot that duplicate others. The worst offenders are the spirit levels for 59p! The iPhone dev kit includes a spirit level as an example project. At least the fart machines have ben programmed and recorded.

Pocket God is an excellent game, well worth the original 59p.
uRemote is also good for 59p.
For anything else, I usually have a laptop on me, so the wealth of the internet supersedes my 'touch...

DOOM - it's a classic. Don't argue - just download!

NOVA - a complete rip-off of HALO, so if you like HALO you're going to love this.