MACMonitor is a Mac OS X Cocoa application that displays AirPort connection information in convenient manners. Most useful for testing roaming Wi-Fi setups it can display both the SSID and the MAC Address of the connected access point. Signal/Link status are also displayed. Additionally, it can display this in an always-on-top window or via the dock menu. A menu extension is now included, which displays more information in the menu bar.
Mac OS X 10.1 or later, AirPort card (including AirPort Extreme)