Ridski

Oh boy... I can't remember which of these I paid for, so, no prices, and they fluctuate anyway.

Facebook - Because it's blocked at work
The Weather Channel - Because I have no windows
Twitterrific - Also blocked at work
SportsTap - Almost up to the minute results for Football, NFL, Tennis and many other sports in one app. Football covers most Euro leagues, US and some South America, UEFA Cup, Champions League and the World Cup.
Ebay - Allows you bid online from your phone.
LBC - Like Classic FM, direct access to LBC Radio in one button, can go from there to XFM and many other UK stations.
PhoneFlicks - Though it changed for some reason and now doesn't access my NetFlix account...
Amazon.com - More online shopping
Emer Radio - I love listening to cops in Oregon, too.
Moviefone - What's on near me?
IMDB - Who's in the movie I'm watching and where the hell did I see them before?
Timer - For counting down
StopWatch1 - For counting up.
CameraBag - Excellent picture processing app that allows you take pics from your phone and make them look like a whole bunch of different styles, Magazine, 1973, Polaroid, etc.
Free Booze - Finds all the happy hours and free nights in your area.
Radio - About 1,000 stations worldwide, including countless online-only stuff.
iVideoCamera - For about 99c, turn your iPhone 3G into a 3GS. Pretty good quality video from your regular still camera.

That doesn't count the games I also have, but then I don't really use it to play music, so I have more room for the other crap.

Englishmum

Here's another one which is very, very useful (but for girlies only!)

Period Tracker (pretty obvious what it is, but you can add notes - so is good if you need to see your gynae, charts ovulation dates etc. and date of next period as it tracks your cycle).

I'm in Singapore and it's great being able to use Skype to make phone calls the UK and US free of charge if I'm in a Wifi zone....

DeadVim

I like the one that alerts you when you are holding it wrong.

It's a masterpeice of design.

emailrob

I'm giving my vote for Slacker Radio, for those of us with iPods and not iPhones

Its the first subscription I've paid for $5 a month. It allows you to "cache" (read download) different stations by genre. Unlimited skips, faves etc. Like Pandora, but I prefer Slacker for their variety and caching is its USP for me.

I cycle to work quite a lot, and it gives me a large array of genre specific "stations". Current hits, 90s, country (erm no) etc etc.

I have a very high bar for paying for stuff and am hard to please and Slacker wins my vote.

nethead

Don't know if anyone already posted this one, but my fave is Google Sky Map. Don't have a iphone have a 'droid.
http://www.google.com/sky/skymap/

emailrob

Agreed, tis awesomeness