Not sure if any of you guys use it or even know about it, but Skype is a relatively simple program that works on Mac or PC and allows you to not only Chat like AIM or Yahoo Messenger, but actually talk over IP. You can use it as a long-distance phone, too, and even call a real phone number.
Until the end of the year, calls to cell phones and land lines are free, and then I believe they will get a small charge after that, but all Skype-to-Skype calls will always be free. If you have a vid web camera on your computer, you can actually see who you are talking to, and visa-versa.
There is a form field in your User CP (Control Panel) that allows you to fill in AIM, Yahoo Messenger, ICQ, MSN Messenger and Skype screen names to contact other members. Under User CP, click the Edit Profile section of your controls, and then fill in the required fields. A small icon under your screen name will appear...
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...to show if you are online or not, and have those services available.
Pretty neat feature, actually.