Oupost was one of the many firewall software I used in the past (right now using Comodo, also free of charge). Agnitum firewalls have always been known by their robustness and pro-active security so the free version shouldn’t be any different.
This one will be on my “To-Try List” in place of Comodo after I reinstall Windows (in a near near future).
Link: Agnitum Free Firewall
Download: Agnitum Oupost Free Firewall