Huge!
See the wilders threads on comodo, I am not saying comodo is the best firewall out there but in leak tests it can nearly pass all of them! Plus it is easy to set up and run.it is still a beta but is now stable and they have reduced the memory footprint.
I was recommending filseclab as a free firewall to my posters at Tech Support Guy, but this firewall does very poorly in leaktests, and now that Comodo is more stable I have switched to comodo over filseclab as ZA is also a bit of a resource hog!
I actually use Jetico!
However, all these firewalls are not bad, just because they don't pass a lot of leak tests doesn't mean they don't block ports and aggressively challenge applications requesting access, but some of the newer firewalls like Jetico and others like outpost are employing HIPS ( Heuristic Intrusion Prevention Systems )like qualities into the firewall which makes them very aggressive!
Basically if a firewall has HIPS capabilities and can block most of the leaktests then these firewalls will be able to meet all types of threats which mostly aren't even in the wild just now!
In the actual leaktests ZA free is one of the worst firewalls tested.
The top four firewalls as from the last tests were jetico, Look N Stop and Outpost joint 2-3 and ZA pro!
see this forum below on firewalls !
Member of ASAP Alliance of Security Analysis Professionals
under the name khazars