ZoneAlarm Pro, a personal firewall, is free today. Details at zonealarm.com.

Personally, I’m a Mac guy and both OS X and XP/Vista have a firewall built in, but ZoneAlarm does offer some better granularity and access control. When I was a Windows guy, though, my personal favorite was Sunbelt Personal Firewall (formerly Kerio, formerly Tiny). Yes, I am anal retentive enough to restrict my browsers outbound access to ports 80 and 443…

On the Mac, I do like Little Snitch for outbound access control and monitoring.