As others have mentioned, GlassWire is quite simple and lightweight. Unfortunately, while it’s absolutely invaluable for monitoring changes to the system (DNS changes, application changes (becoming unsigned, etc.), webcam and microphone changes, etc.), its firewall is too simple compared to Little Snitch. Unlike Little Snitch, which lets you choose which domains and ports an application is and isn’t allowed to connect to as it attempts to make those connections, GlassWire only allows you to either allow all connections an application makes or deny all connections an application makes. NetLimiter¹ is much closer to Little Snitch in this regard (example²).

Personally, I’d recommend using a combination of GlassWire for monitoring system changes and NetLimiter for the firewall.

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¹ — http://www.netlimiter.com/products/nl4

² — http://www.netlimiter.com/Files/ImageGalleries/netlimiter-4-...