One of my favorite things about the iPhone is that I can stay connected to my friends and coworkers no matter where I'm located. One of the best examples of this is the IM+ Lite application. IM+ is an instant messaging application which supports AIM, Google, Yahoo, MSN, Jabber, ICQ, Myspace, and Facebook. I personally only use AIM, Google, and Yahoo, but I'm sure all the other services work the same.
When you first boot IM+ you'll be asked to add an account. The process is simple and quick, and will get you chatting right away. Then you'll see your Contacts or Buddy List, on the first tab. You can slide to delete contacts, click them to start a chat session, or click the "i" button to get info like their profile.
The Inbox is the next tab, where you can see what open chats you have, and whether there are any new messages. If you click one of the names, you'll go to the chat screen. It's a simple layout, text on top, type pad on the bottom, and a few buttons to move back to the Inbox, or to close the chat. There's also a smiley menu which allows you to insert graphical smilies, assuming the person on the other end can support that.
One great thing about the chat screen is you can swipe left and right to go from one person to the next. This makes it really easy to maintain a few chat sessions at once, something I'm almost always doing.
The third tab is Status. You can set yourself online, away, invisible, or offline (which disconnects you). Pretty straightforward, pretty useful. Finally, there's a Settings tab, and an About tab. The settings include which accounts to enable, and a whole host of other options about how the app works.
When it comes down to it, a chat client is about ease of use, reliability of connection, and the ability to use multiple chat clients. IM+ Lite covers all 3 bases, perfectly. In the full version, you can send pictures and do voice chat, along with some other add-ons. The free version covers all the bases though, and it's a must-have if you want to stay connected on the go. There's a few other chat options out there, and some of them are decent, but IM+ is by far the most full-featured and easy to use.