A couple of times I've been near an available network and unable to connect because the MacBookPro picks up too weak a signal. This was especially frustrating when PC users were sitting there in the same room using the wireless connection that I couldn't even see. This weekend, I managed to get on but had barely one bar and the connection kept quitting on me.
So I found this little device called WiFire
It is $79 and plugs into a USB port and then takes over for the internal airport connection. Sitting in my living room I pick up two other WiFi networks, both passworded. With WiFire I pick up nine networks besides my own, three of which are unprotected (which WiFire's connection window shows at a glance.) One of those is strong enough that WiFire can maintain a connection to it, but two of the other unprotected ones do blink on and off.
The device is not without its flaws. It perches, rather than clamps, onto your computer. And although it swivels to pick up the best angle, its cord tends to pull it back to one direction.
What I really hope to be able to do is pick up my Mom's wireless connection when I am up in Washington, but even though she is Line of Sight I think it will be too far away. Still, for getting a more secure connection to legal but weak signals, this will be great, and I'm glad I got it.