There are a ton of iPhone apps that I use on a regular basis. If I had to pick out five, these would be it.
Tweetie is a great Twitter client for the iPhone. It supports multiple Twitter accounts as well as Ping.FM and Instapaper. You can do things like look at @replies and direct messages in a seperate window as well as favorite tweets right within the app. Another cool feature of Tweetie is that it uses the GPS in the iPhone 3G and the iPhone 3GS to find people nearby you who have tweeted recently. There is also a version for the Mac. Tweetie is an app that I could not live without.
BeeJiveIM is a multi network IM (instant messaging) client that supports AIM, MobileMe, Windows Live, Jabber, Yahoo!, Facebook, Myspace, and ICQ. Even though this app has a high price tag, it is most certainly worth it if you use some to most of those networks. BeeJiveIM also lets you customize many different parts of the app such as the chat background as well as the color of the chat bubbles. I constantly use BeeJiveIM almost 24/7. In iPhone OS 3.0, it supports push notifications. It is most certainly worth the price for me.
Pixelpipe is an application that will upload your pictures and video (iPhone 3GS only) to a ton of many different sites. The list is so huge to even start to put here. This app requires a free account on thePixelpipe website. I use this application to upload almost every picture that I take with my phone. It is extremely convenient to be able to take a picture and upload it almost right away to the web.
The Wordpress for iPhone app allows me to update any of my blogs that are hosted on Wordpress (hosted on wordpress.com and wordpress.org blogs) right from my phone. Most of this post was in fact written on the Wordpress app. The app can get a little wonky at times but otherwise it does its job. It lets you add photos taken from the camera, save drafts to the phone, edit drafts saved on the server, add categories and tags, and publish posts.
Shazam is a great app for showing off all that your iPhone can do. If you like a song and don't know the name of it, simply launch the app and hold it up to the speakers and Shazam will identify the song that is playing and it will give you some links to places that you can find it. Anytime I show someone Shazam, they always want to see it again. There are only a couple times when Shazam hasn't been able to identify a song either because it was coming out of crappy speakers or that is was a lesser known song like an indie song. Best of all, Shazam is FREE!
These are just some of the many iPhone apps that I use on a regular basis. I use a ton more but these are my favorites. Have a favorite iPhone app? Tell me in the comments. Want me to review an iPhone app? Send an email to email@example.com.