bwm-ng (command line bandwidth monitor)
bwm-ng is a great little command line bandwidth monitor. HUGE fan. Its available with 
most all distros so use your favorite package manager to add it. Works on all *nix distributions including the Mac too.
bwm-ng home page: http://www.gropp.org/?id=projects&sub=bwm-ng
On the Mac it works great with a little application called GeekTool (will cover more later) with the following options:
/Users/derek/Applications/bwm-ng/bin/bwm-ng -o plain -c 1