Besides  email,  safari,  phone,  Messages,  Calendar  and  ipod which I use all day and night, its a part of me, some other tools I use daily:

Things - a great to do list manager that works and syncs with the desktop app or works independently.

Now Playing - my favorite app for checking movie times whereever I'm at

AIM - for connecting while on the road with my team

Tweetie for twitter

 Top Floor for highrise access on the road

 GroundWork for basecamp access on the road

Facebook keeping in touch with friends and family

 QucikOffice for working with Word, Excel, and PowerPoint files and editing them

Shopper for managing my shopping lists

1Password for your Mac and iPhone to track ids/passwords for your accoutns

 Amazon Mobile for ordering or checking reviews on products, amazing example of how an app can be used with web data

 Are My Sites Up - for monitoring web site status

Sportstacular for tracking my favorite sports teams and upcoming schedule

Jaadu VNC for accessing machines on the network

Webex Meet for participating in Webex on the road

Ego - track Twitter followers and GoogleStats

 Photos for quick presentations, save your presentation as a series of images, great for a quick 1 on 1 presentation or review with a client