Our friends over at the Mactipper.com have put together a quick tutorial on how to get your mac to just join the best open Wi-fi Available.
Go to the network system preference pane.
Open the Airport section by changing the “Show:” menu to “AirPort”.
Click on the “Options…” button in the lower left hand corner.
Change the menu at the top of the Options window to “Automatically join an open network”.
Hit “OK”. Then, click on “Apply Now” in the lower right hand corner.
If you want to see him walkthrough this in a screen cast jump over to his Mactipper.com and check it out.