I prefer iStumbler. They also have a neat widget but it has to have the main program running in the background. I have used MacStumbler too, but iStumbler is my favorite. It does BlueTooth as well.

Widget screen shot
The widget connects to iStumbler to display the network names for all visible and connected networks. The samples are color coded to the network type.
The Sweetest part is that it shows networks which may interfere with each other as overlapped in the display.
It is still free, but a pro version is in the works