The blurb Radio Silence has at the bottom of their page even reads "A hassle-free, and cheaper, alternative to its cousin Little Snitch." 

StillStackinAfterAllTheseYears

Indeed. That was the first one I learned about. It's fully featured for power users.

Yet another option is [TripMode](https://www.tripmode.ch)

nullama

[LittleSnitch](https://www.obdev.at/products/littlesnitch/index.html) is another great option if you want more customisation and control.

davidw

I wish they would let me buy it with Bitcoin.

chungkingexpress

I like the fact that I can block any application from accessing the Internet on a mac. Really this should be included in the OS, but it isn't for some reason.

Seems quite nice and it's just $9 for a lifetime license. I think it's worth it.