MarcoPolo exists because the Mac OS X concept of a "location" is a collection of settings for your computer's networking devices. Switching your current location is much easier than changing all those individual settings, yet you still have to explicitly switch to the correct location each time.
MarcoPolo brings context-aware computing to your portable Mac computer. It allows your computer to determine its context through gathering evidence from your environment (evidence sources), using flexible rule-based fuzzy matching to make an educated guess (rules), and then performing arbitrary actions upon changing
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."
- Implemented ScreenSaverStart action (Trac #142)