PC Tonic

Comodo Firewall Pro ~ Comodo Group
*****
Available for: Windows Vista/XP (version 2.4 is available for Windows 2000)
Download: home page | Download.com | FileHippo

PC Tools Firewall Plus ~ PC Tools
****
Available for: Windows Vista (32-bit)/2003 Server/XP/2000
Download: home page | Download.com

ZoneAlarm Free Firewall ~ Zone Labs
****
Available for: Windows Vista/XP/2000
Download: home page | Download.com | FileHippo

Sunbelt Kerio Personal Firewall ~ Sunbelt Software
***
Available for: Windows XP/2000
Download: home page | FileHippo

Outpost Free Firewall* ~ Agnitum
**
Available for: Windows 2003 Server/XP/2000
Download: home page | FileHippo

Sygate Personal Firewall ~ Sygate Technologies**
**
Available for: Windows 2003 Server/XP/2000
Download: FileHippo

[Notes]
* Outpost Free firewall from Agnitum is no longer officially supported. The version 1.0 has been out since 2002 and is completely obsolete now.
** Sygate Technologies has been acquired by Symantec makers of the Norton range of products. Sygate Personal Firewall is not officially available any more.

[General Note]
You should never use more than one firewall on a single computer, as it will slow down your computer and can even cause driver conflicts.