ProgCop is an Open Source application firewall for your Windows machine.

ProgCop is explicitly designed to make blocking applications from going online in a more straightforward process. ProgCop features a real-time view of processes that are connecting to the Internet. It then gives you the option to block them with a single mouse click. ProgCop will be an essential addition to anyone's privacy toolbox allowing you to prevent apps from phoning home without being overly complicated.

ProgCop uses Windows Firewall API to block outbound connections for applications. Because ProgCop uses builtin Windows APIs, no kernel-level drivers are needed, which keeps your system stable. ProgCop is meant to be used together with the built-in Windows Firewall to block outbound connections.

Note that ProgCop needs admin privileges to run (because it's using Windows Firewall API). When starting ProgCop, it will ask for the permissions.

ProgCop Features:

Lightweight - does not run in the background
Stop apps from phoning phone
Exposes apps and executables making connections
Block or unblock with a single click
Safe - does not make your system unstable
HiDPI support
Non-intrusive operation
IPv6 support