MarcoPolo is a Mac OS X concept that a "location" refers to a collection or settings for your computer's network devices. Although it is easier to change your current location than all the individual settings, you must still explicitly switch to the correct one each time.
MarcoPolo brings context-aware computing right to your Mac computer. It allows your computer's context to be determined by gathering evidence from your environment (evidence source), then using flexible rule-based fuzzy matching (rules) to make an educated guess (rules), before performing arbitrary actions when the context changes (actions).
MarcoPolo's concept contexts is more than just a generalization of a place. A context could be what you are doing or what is happening around you.
MarcoPolo remains silently in the status bar at top of screen (right-hand corner of the menu bar), where you can configure it to your needs.
Note from the developer: "Sadly, I have not had time to maintain MarcoPolo for the last few years, and it is becoming less functional. For instance, the WiFi evidence source does not work on Mac OS X 10.6 or 10.7. Much of it still works, but you should consider this software unsupported."