Locations ($19, by @warwick) lets you change your system’s settings based on which network it’s connected to.
When I take my computer to work, it complains because it (understandably) can’t find the wireless drive I have Time Machine set up on. Every hour, OS X reports that Time Machine failed to back up. I could just turn TM off at work and turn it back on when I get home, but then I run the risk of forgetting to turn it back on. So I have Locations set to automatically disable Time Machine when I get to work and then re-enable it when I get back home. I have Locations set up to do a couple of other things, but the Time Machine problem was the one that annoyed me the most.