Obviously everyone uses them for different things, but if you have an iPhone, share your favorite apps and preferably a link to download/read.
My top five are:
Convertbot: Very nice app for converting almost anything you can think of...Time, Temperature, Mass, Length, Data, Currency, Area, Volume. I estimate at work so being able to convert various measurements comes in pretty handy.
iVideoCamera: If you don't have the 3GS phone, you can still have video capability. Works perfectly.
Accuweather: I've tried a couple of the other weather apps and like this one the best. Easily scroll between different locations to keep an eye on weather where family is at, vacation spots, etc.
TextMinder: I love this one. If you have something important coming up, text yourself a reminder. On the day and time you pick, you'll receive that text with the reminder. Great for anniversaries, appointments, etc.
Dragon Dictation: Short texts while driving aren't usually a big deal if you're careful. But if you have a lot to say, this app is great. Press the button, it'll record your speech and translate it to text. It then gives you the option to paste it into a text message.