Spoofing MAC address in OSX Tiger
The MAC address (media-access-control) is a unique hardware identifier hard-coded into every ethernet/wifi/airport card. Some wifi providers use MAC address filtering as a security measure, i.e. they have a list of allowed MAC addresses, and anyone not on that list can't connect to the network.
Sometimes you may want to bring your personal Mac to work/uni to fix some problems, etc. But if your wireless and wired LAN connections are both MAC filtered, you need to request the IT department add the MAC for your Mac so you can connect. You can find out the MAC by going to System Preferences > Network > Configure > AirPort. The AirPort ID is the MAC and looks something like 00:17:f2:ab:03:05. You'd have to send this to the IT department and ask them nicely to add it to the allowed list.
Alternatively, if you're in a hurry, you can always just replace this MAC ID with one from your work laptop or PC which is already on the list. This is called MAC spoofing. So once you find out the MAC from your other PC (for Windows, open a cmd prompt and type ipconfig /all), go to your Mac, open up a terminal and type 'sudo ifconfig en1 lladdr new_mac_address', then 'sudo ifconfig en1 down' and finally 'sudo ifconfig en1 up'.
Note that en1 is the system name of the AirPort adapter on my Mac. To find out which interface is which, go to About this Mac > More Info > Network. The top-right pane should show you all your network adapters with their friendly name (AirPort, Ethernet, Bluetooth, etc) and the BSD Device Name (en1, en0, fw0, etc).