We've previously mentioned locations managers like Location X ($20) and NetworkLocation ($15). These are tools that can change a variety of settings depending on your location (e.g. SMTP server, default printer, etc.). This is very convenient for mobile users moving between work and home, etc. MarcoPolo is a new entry in this category and adds a number of cool features that we haven't seen before, including "fuzzy logic" rules for setting the location based on matching a variety of different criteria (wifi network, IP address, etc.) and USB and Bluetooth support (as criteria). The USB detection is a great feature if, for instance, you have one model of mouse and/or keyboard at work, but use a different set a home.
Best of all, MarcoPolo, unlike most of the other utilities, is open source and a free download.