Yahoo Messenger is an instant-messaging application that has been around for many years and has been a popular IM tool on different operating systems. Now that there's a port to iOS, Yahoo Messenger can be ubiquitous. It's available from iTunes and several other sites, and easy to install.
In Yahoo Messenger you can add contacts by e-mail address or invitation, break them into groups, see when they are online, and message back and forth with multiple sessions active at once. Yahoo Messenger goes beyond basic IM by adding integration with the iPhone and iPad camera. You can launch the app and stay online, with the usual notification options available when someone messages you. You can hide some or all of your activities using various stealth options, too. A history feature lets you review previous conversations.
We've been using Yahoo Messenger for over a decade now, and the port to iDevice made a few years ago just made this platform even more useful. While there are a lot of IM apps out there, few have the install base of Yahoo Messenger, and it remains the one IM app we return to over and over, mostly because we're used to it and have all our contacts organized and active.