Altho certainly not free, I would go with this 9as a matter of fact, I do
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3033708375&category=3706
that and running syslog on one of your LAN pc's and you have a mighty fine security solution on the cheap. Can do content filtering, create in/out ACL's, even use 3des VPN
Aside from mine cutting thruput on my 8mb cable connection (down to about 3.5mb :-(, I luv mine.
Oh, the LAN ports are only 10mb so, u may want to pick up a 5-8 port 10/100 switch for your LAN, then plug one of the LAN ports from switch to LAN port of Firewall.
Not free, but relatively cheap for what u get. Search the hot deals archives, you used to be able to get them for $20 + shipping (then needed to upgrade the users/vpn for like $30 or so).
OOps, I just caught the SOFTWARE, well, this is better then software based stuff (software running ON TOP of your OS for those smart alecs who will say that a hardware based firewall is run on software (which it is, just embedded, not on top