People have already started making lists of their favorite iPad apps, so I guess it's time to list 6 iPhone apps that I use almost all the time on my iPhone (games count as bonus):
Mail is like the water of the intertubes. I am an email junkie, so Mail takes the first place in this list. I have all my email accounts synced to the iPhone so I am always up-to-date.
Tweetie
Tweetie is the Twitter client for the iPhone. Its features and unique UI ("Pull down to refresh..." - awesome) makes every other Twitter client just not good enough. 'Nuff said.
Reeder
If you are also a Google Reader junkie like me, you will find what you need in Reeder. Among others, Reeder features Instapaper and Readitlater integration, beautiful and useful interface and in-app browser. The definition of 'money well spent'.
FreeMemory
Although not available on the App Store anymore (Apple removed those kind of apps some time ago), it's pretty handy: Once you get a low memory warning from an app, you don't have to reset the phone, you just fire off FreeMemory and does all the dirty job for you.
Spotify
Tricky to get (both the Premium account and the iPhone app are not available in Greece), but once you use them you can never go back. Offline mode is essential for having some music on-the-go while driving.
Deliveries
Buying stuff online require tracking them and Deliveries does exactly that. It supports almost every delivering service, and yes you can track your package in Google Maps.
Bonus 1: Plants vs Zombies
PvZ is pretty unique. Not only from gameplay point of view (mix of tower defense with resource management and card game features) but from the fact that anyone can play it and get addicted easily. And I mean it: anyone.
Bonus 2: We Rule
The new craze! We Rule is like Farmville but better. You have your own kingdom, you build businesses, houses and farms and you order things from friends' businesses, acquire taxes from houses and harvest your crops. Caution though: it's extremely addictive.