some really nice IM clients (Jabber/IRC/ICQ/AIM/MSN/etc.)

• Adium X - a really nice Jabber/ICQ/AIM/etc. client for OS X
• Colloquy - a quite nice IRC client, IMHO preferrable to most others
• Fire - another multi protocol instant messaging client for OS X
• Xchat Aqua - a port of Xchat (an X11 IRC client) to OS X (without X11)