ICQ ('I seek you') is a user-friendly Internet program that searches for your friends and associates on the Net, alerting you in real time when they sign on. With ICQ, you can exchange messages and files, engage in real-time chat, play games, and more. ICQ now stores your contact list on the server, a new ICQ global hot key can be set in the preferences, better AppleScript support and much more.
Real neat product, love the text-to-speech especially, but sorely missing the IRC-style history pane from ICQ2000 for Windows (copied in the recent Licq for Linux too).
Very stable
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Summary
He is a very stable messenger for OS X and you can very use it to taste in OS 9 or OS X when you prefer.