I currently use linux 2.4 and iptables with a script originally created by fwbuilder 1.something. I have a couple of NAT rules so that external users are forwarded to internal servers based on the port they hit, for example:

external addr:21 -> 192.168.0.2:21
external addr:80 -> 192.168.0.3:80

My issue is that I want to host multiple web servers on port 80 using host headers so that:

external addr:21 -> 192.168.0.2:21
www.domain.com:80 -> 192.168.0.3:80
ww2.domain.com:80 -> 192.168.0.4:80

I haven't been able to find anything telling me if that's even possible with my setup, much less how to implement it.

If linux/iptables can't do this, anyone know of a free os/firewall that can?

well that will cover forwarding and such, but there's also the fact that you want to do it via a firewall. this isn't really something you should want to do conceptually. "traffic on port 80" is very different to "viewing a website on another server". elect a webserver to be the master one (maybe even a dedicated low end box if need be) to proxy all port 80 traffic, interpret it within apache and then act accordingly.