Few users are aware that they can use the wireless card on their Macs (or PCs) to create what is a called an ad hoc/computer to computer network - enabling you to extend a single internet connection to various computers/devices without the need for a standalone wireless router - your wireless card acts as a wireless access point.
Ad hoc networks are useful in a hotel room when you may want to extend the single hotel internet connection to your other devices or to a colleague's laptop. Even if you do not have internet access, you can use an ad hoc network to share files between computers.
Setting up a Wi-Fi ad hoc network on OS X
1. Click on the AirPort icon in your menu bar. From the AirPort menu, select Create Network.
2. Click on Show Options and encrypt the connection by entering a password as described in the dialog box.
3. Click on OK.
4. Each user on your ad hoc network should now select the network that appears in the AirPort menu and enter the specified password.
That's it - you can now share a single internet connection across various devices, and you can share files cross all devices on your ad hoc network.
Note: in order to be able to share files across computer, you must make sure that file sharing is turned on - In System Preferences, click on the Sharing icon and then on the Services tab. Turn on Personal File Sharing to share with other macs and/or Windows Sharing to share with windows PCs.
Windows users can also create an ad hoc network, it is only a little more difficult to configure - for windows instructions, see Making the Wireless Home Network Connection in Windows XP Without a Router.