With Yahoo! Messenger Mac OSX you can send instant messages to your Windows and Mac friends.
You can even uze Yahoo! Messenger with your mobile phone, send files to friends and receive notification when they have been received, use a webcam to share live video with your friends, send files to friends and receive notification when they have been received and express your feelings in a conversation using Emoticons (little animated smiling faces).
Freeware, for Mac OSX