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TCPBlock is free and can be downloaded from MacUpdate, however TCPBlock doesn't appear to work with El Capitan and is no longer being updated.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