Apple - Which free alternatives to Little Snitch can I use to block internet traffic per applications?

TCPBlock

TCPBlock is a lightweight and fast application firewall … you can prevent selected applications on your computer from opening connections to the network.

TCPBlock is free and can be downloaded from MacUpdate, however TCPBlock doesn't appear to work with El Capitan and is no longer being updated.

PF Firewall

OS X comes with the PF firewall, which can be configured to block all connections to certain servers in all apps. Blocking all connections from a single app is not possible as far as I am aware without external software.

You can use IceFloor as a GUI for the PF firewall.

LuLu is a great free, shared-source, macOS firewall that can block unknown outgoing connections, unless explicitly approved by the user. It runs on OSX 10.12+ and runs well on macOS Mojave. It can be configured to allow or block Apple signed binaries and has clear GUI based rule set that can be edited. Whilst it is free users are encouraged to support its development - which seems fair.

Use Radio Silence. Very simple application, but does the job nicely, however it costs $9.