hold down option and click your airport menu item. Now mouse over any entry listed there and a pop-up will show you channel, frequency, security, mode (b/g/n etc.), RSSI (signal strength) and BSSID (the MAC address of the WAP).
edit: I've also tried airmoose but it seems to endlessly scan. I'd also be interested in a stand-alone app that would display the above data and maybe more in a neat table-column layout.