I've been traveling a lot lately and, as a result, spending a lot of time tethering my Mac to my iPad and iPhone. Trip Mode is a Mac App that can turn off selected applications' access to the Internet. For example, I share multiple Dropbox folders with multiple people. If one of those people starts dropping some large files in Dropbox while I'm tethered, the Dropbox app will chew through my mobile data cap pretty quickly. Trip Mode lets me turn off Dropbox and any other applications I don't want accessing the Internet while tethered. The idea is simple, the execution is outstanding. When I return to WiFi, Trip Mode turns itself off and I can go back getting all my applications on the Internet.