Alternatives

Honest comparisons with the apps you might use instead.

Charles Proxy Alternative
Charles is an HTTP debugging proxy. Depending on whether you need request contents or just connections, there are better-fitting tools.
GlassWire for Mac? The Best Alternative
GlassWire doesn't run on macOS. If you liked its per-app traffic graphs on Windows, these are the Mac options.
Hands Off! Alternative for Mac
Hands Off! was a popular Mac firewall from Metakine, but it's no longer sold or updated. Here are the current replacements.
Little Snitch Alternative for Mac: See Every Connection
Little Snitch is a firewall with an excellent network monitor. If you mainly want to see what your Mac connects to, rather than write rules, LightAtom is a lighter fit.
LuLu Alternative: When You Want to See, Not Just Block
LuLu is a great free firewall. It tells you when an app connects; it doesn't show how much data apps move or where it goes over time.
Radio Silence Alternative for Mac
Radio Silence is a simple, quiet firewall. LightAtom is a monitor. Which you want depends on whether you're blocking or understanding.
Wireshark Alternative for Mac: No Packet Capture Needed
Wireshark is the standard packet analyser, and overkill for “which app is talking to what?”. Here's when to use each.