There are dozens of applications and services on your computer which connect to the Internet. These applications/services often run in the background and send off various data to remote servers. Most of this background data transfer is harmless (e.g., iTunes looking for app updates, your RSS reader refreshing your feeds, Mail checking for new email), but there may be applications or services accessing the Internet without your permission, or even without your knowledge.
This is why a good firewall is indispensable. While OS X comes with a built-in firewall which does a good job of giving the end user control over much of what transpires in the background, sometimes the built-in firewall just isn’t enough. In an age of growing concerns over information security, it is vital to know what applications and services are accessing the Internet and what servers they are accessing. For full control over and monitoring of your computer’s Internet activity, nothing beats Little Snitch.
I’ve used Little Snitch for years. It’s one of the first things I install when I get a new computer (or configure one for a friend), and I wouldn’t use a Mac without it. For anyone interested, now is a good time to get Little Snitch as it is on sale (via MacUpdate). Usually $35 for a single license (or $69 for a family license), Little Snitch is just $19.99 for the next two days. If you don’t already own a copy, make sure to grab it before the sale ends.