If you have an AirPort card on your Mac -- all of the current notebooks, iMacs, and Mac minis do -- you can turn your Mac into an AirPort base station that can be used by other Macs and PCs in your immediate area. Once you are connected to the internet, open up System Preferences and select the Sharing icon. Once the Sharing panel opens, select the Internet tab. Check the "Airport" option in the list of things to be shared and then hit the "Start" button. Your panel should look something like this right before you hit the start button:

The Sharing panel in System Preferences right before AirPort sharing is turned on

If you want to get really fancy, you can click the "AirPort Options..." button to do stuff like enable encryption of the signal and to require passwords to use the network.

Once you hit the "Start" button, you'll notice that your AirPort icon in the system tray changes to indicate that you are now broadcasting. Once you are broadcasting, other computers can hook up to the wireless network created by the AirPort card on your Mac.