The gumstix Overo boards have a built in Serial<->USB IC. This is pretty neat because you don’t need to worry about buying a USB Serial cable, or buying/building TTL/RS232 level converters. Setting it up on a Mac requires you to install a driver though, and it took a little searching to figure out which driver I needed, so here are my notes.
Firstly, on a Tobi board the serial console is located on the opposite corner to the Ethernet socket:
You need to download and install the following driver: http://www.ftdichip.com/Drivers/VCP/MacOSX/FTDIUSBSerialDriver_v2_2_16.dmg.
Install a terminal client (I used CoolTerm). Set the Baud rate to 115200, and you should be good to go: