I'd just like to comment on the reviews pointing out that Marco Polo and Location X are more flexible. It's true. They are. They also represent the same excellent value for money (all are free). But the thing to remember is that all three apps do what they do slightly differently - so download them all and find out which one fits your needs most closely.
Marco Polo and Location X both permit fuzzy identification of your location - you they use an evidence based system, weighing up USB, Bluetooth, Wifi, Ethernet etc. before deciding on a location.
Locamatic is simpler. It just checks to see which ethernet or wifi connection your Mac is connected to and sets the location accordingly. Why? Well, I couldn't see the advantage of the fuzzy method. If you can then Marco Polo and Location X are both excellent alternatives.
Marco Polo and Location X are both applications. They can be set to load at startup and not trouble you with their existence, but you need to set them from their own application windows.
Locamatic is a Preference Pane. Why? Well, I figured that you set everything else from the Preferences application and so it'd annoy me if Locamatic was any different.
Locamatic (just so that there is no confusion) is still being developed, I haven't abandoned it. It configures proxies, homepage and default printer on the basis of connected ethernet or wifi connections. And it uses no memory or CPU until a connection changes - and then it's active for a few seconds before disappearing again.
Now go! Download all three!