PC Tools Firewall Plus
Full user review
Very light on system resources, stable, full-featured, clean interface, relatively easy to use, doesn't nag too much, it scored high on Matousec Firewall Challenge!
Not really a con, but I wish there was a way to close/resize the white space ( that displays options like Allow/Block ) on the right side of the application. Maybe integrate the options in Right click menu in next update?!
I had trouble accessing the internet when I first installed it (on Vista), even though I clicked on Allow for Firefox. Turns out I had to Allow "Userinit Logon Application" also for it to work.
I like the fact that It has Advanced Rules to configure inboud/outbound connections. I enabled: "TCP/UDP: Authorize most common internet services (Vista)".
I was using Online Armor before this which is also a good firewall but the interface was less intuitive, and it nagged a lot unlike this one.
To sum up, Firewall Plus seems like a solid firewall.. Give it a try, you won't be disappointed.