Windows only: Keep your PC safe with Comodo Personal Firewall, a free security program designed to block outside intruders and keep your PC from "leaking."
Unlike the free version of ZoneAlarm, Comodo recognizes thousands of known programs as safe, meaning it won't bombard you with pop-up warnings for those programs. It also stops leaks--malware that tries to trick a firewall by masquerading as a legitimate program. Thus, Comodo should prevent your PC from becoming a "zombie" PC in the event some malware slips in.
I find Comodo less intrusive and more accommodating to networked PCs than the free version of ZoneAlarm, which is why it's now my preferred firewall. These days, no PC is safe without one, and Comodo offers excellent protection. The program is free for Windows.
I'm trying it out here at work and seems nice.