Get more out of your Airport card with AirRadar
AirRadar color-codes the available networks, so you can quickly see which ones are open or protected, and what their signal strengths are. You can tinker with colors, view a signal-noise graph, and add networks to your favorites (there's a color for that, too.) Just as importantly, you can disconnect from a network without toggling your Airport off and on. It also fixes those auto-join problems by letting you set more specfic parameters for when it should get you onto a network.