There are few apps that I like as much as Little Snitch -just like I made it clear very recently-, so it seems to me that you are facing a great opportunity to get one of the few apps that I cannot conceive of a Mac without. The sale lasts until February 3.
Let's review the virtues
Obdev's masterpiece is a advanced firewall that allows us to create rules with different options to block or allow the different connections that are made to and from our Mac, thus being able to manage the entire network at will. Although Mac OS X has a firewall by default, compared to Little Snitch it is evident.
The regular price of Little Snitch 3 is $ 35, while now it is available for less than 18, a price that for example is lower than other apps like Tweetbot and that from my point of view makes the purchase of this application totally essential if we like to have a minimum control over our Mac.
Of course, it should be noted that it is an app for those of us who like control everything. If you want a non-intrusive and efficient firewall against external threats, OS X's is free and you will have plenty.
Link - odev