MarcoPolo is a free software for Mac OSX based computers that allows you to change a variety of settings (for example, default printer, opening a file or document, setting default SMTP email server for mail.app, etc.) on your Mac based on your location.(via)

The software tries to determine your location based on the following devices/protocols/logic:

Discoverable Bluetooth devices
Assigned IP addresses
Attached USB devices
Visible WiFi networks
Fuzzy logic- mixture of bluetooth, USB, WiFi, etc

Download here