Ever since it was possible to tether a computer to a cell modem, it's also been possible to blow through one's monthly or service-plan limit and either run out of mobile data, be throttled to a trickle, or face expensive overage fees. TripMode is the first easy-to-use OS X utility to help with that problem. It could do more, but for $8 (or $5 in its current sale), it does plenty.
When installed, TripMode appears in your system menu bar and monitors for network changes in Yosemite. Whenever you join a new Wi-Fi network or connect to a Personal Hotspot, TripMode activates and blocks all system-level and application network usage. The utility was built as an access whitelist, so all network usage is blocked until you allow it.
You can check boxes next to any activity you want to approve from TripMode's dropdown menu. As new services or software tries to access the network or the Internet, more entries appear in the list. You may be surprised what appears, as many apps regularly poll servers in the background to check for software updates or event updates. TripMode can't populate the list fully initially, because it only "knows" that an app or service requires the Internet when that occurs. The utility's icon turns red whenever an app that's blocked tries to access the network.
Individual software products have limited awareness of the network to which they're connected when you're on a Mac. The iOS operating system and iOS apps typically are more careful about letting you pick and choose what's sent over cellular and what's not. In OS X, Dropbox has a Pause button and CrashPlan, my backup software of choice, lets you blacklist Wi-Fi networks by name. But OS X more or less assumes it can always let apps use 100 percent of available throughput. Photos for OS X is a great and terrible example of that.
TripMode turns on automatically for every new network or new Personal Hotspot mode (such as a USB connection), but you can override the setting and it remembers that override. For instance, connect via USB to your iPhone or iPad to use its Personal Hotspot, and TripMode activates. If you click its switch from On to Off, however, the next time you connect via USB TripMode will remain off. It retains this information for every network to which you connect, restoring whatever state you left it in when you last connected.
TripMode keeps track of data transferred while it's active, though not by network, just cumulatively. Still, it's likely you'll use it mostly to restrict excess usage on a mobile network, and thus its total remains useful. You can view data in the last session, the current day, or the current month.
Sign up for CIO Asia eNewsletters.