Maybe most useful apps to you, but to me I find incredible value and quality in lots of 3rd party apps. Here are some of my first screen apps......
Omnifocus - there is no task management app on any other mobile device even close to this.
The Omni Group - OmniFocus for iPhone and iPod touch
Note2Self - I use this app to leave myself voice memos. The app starts recording when I lift the phone to my mouth. When finished it automatically emails me a copy so I can store it also on my laptop. I have read a newer version will transcribe the voice notes into text.
Welcome to WebIS.net - Note2Self for iPhone
Splash ID - Password protection app, far better than the Palm OS version. Sync from my laptop to iPhone over encrypted wi-fi.
MLB at Bat - Big Met fan (I'm crying on the inside). This app allows me to follow in-game highlights and scores. The 2009 version will offer in game audio broadcasts. If you like baseball, buy this app.
MLB.com At Bat™ | MLB.com: Mobile
BeejiveIM - used it first on my old Blackberry curve. Its by far the best IM client available. Landscape keyboard, awesome graphics, text and email notifications. Since I started using this app, I find myself never logging into IM anymore on my laptop. I do it all from my iPhone.
Beejive - instant messaging anywhere
Evernote - Future PRE owners should start migrating their outlook tasks to this app now. Awesome note take app that integrates real nice with outlook. The iPhone client is very nice.
Evernote for iPhone | Evernote Corporation
Stanza - I have a long train commute each day. Gives me plenty of time to read. This ebook reader allows me to create my own ebooks, download directly from fictionwise, or purchase from ereader.com. Love the interface.
Stanza: a Revolution in Reading | Lexcycle
Pandora Radio - Internet radio at its best. Love it. Please create a PRE version.
Pandora Radio - Listen to Free Internet Radio, Find New Music
AP News Application - I'm a news junkie. This app is awesome. World news, local news, sports, business, weather, entertainment, etc.. All included in an app with an incredible UI.
Wikipanion Plus - I need to do a lot of research for my job. Nothing beats Wiki. This app for the iPhone allows me to download articles to save and view later.
The Wikipanion Experience: Plus Features
X-Plane - I am a amateur pilot. This app is an incredible flight simulator program with a desktop quality. If you like to fly, try this app. Click the link to see the graphics.
New York
Now Playing - App that will show movie times at local theaters, reviews, trailers. Its last update brought Netflix queue management. All your movie needs are done in this app.
NowPlaying - metasyntactic - Google Code
Other apps I use: NY Times, Handbase, Chase on-line banking, Texas hold em poker, Twitteriffic, USA today crosswords, Amazon shopping app, lots others.