Adium supports popular chat networks such as AIM/MobileMe, Google Talk (along with all other Jabber/XMPP-based chat services), MSN and Yahoo! Messenger; it also connects to Facebook chat and other services like QQ, Gadu-Gadu, Lotus Sametime and Novell Groupwise. No, it doesn't do voice or video (or FaceTime), just sweet old text like our forefathers intended.
The Adium open source project keeps chugging along, despite a severe resource crunch and shortage of development hours. If you're a savvy Mac programmer with an interest in the topic, the team would welcome your energy and contributions.