ICQ is probably the most popular buddy contacters/direct-chat applications available.
This one isn't quite as fully featured as the PC version, but for instant messaging with a large installed base of users, it can't be beat.
For more detailed information, including on updates, please read the documentation or visit the home page.
Note: NOTE: Please note that this time-limited free beta version of the ICQ for Power Mac (PPC) may not include certain features and functions that may be available on other versions of the ICQ software.