If I had to choose five essential programs on my Mac, rest assured that one of them is Little Snitch, a magnificent program but unfortunately it is commercial. Do you want a free alternative? Here it is.
TCPBlock is not nearly as complete as its competitor, but it does exactly the same thing: monitor the connections that enter and leave our Mac and then filter them as appropriate.
Requires Leopard and supports 64-Bit.
Download | Aprons