FireBwall is a free and highly modular firewall for Windows. Extensibility is a key part of the program's design, so anyone can create modules for the firewall and use them right away, just by placing them in the appropriate folder.
Despite fireBwall's low version number, the program still has a lot of modules available. The basic firewall allows you to block or allow packets according to their IP address and port, of course, but you also get a MAC Address filter, ARP poisoning protection, some mitigation for DDoS attacks, an ICMP filter, an IP blocking tool, an IP monitor and port scan detector (check the current list here).
Look at these modules individually, though, and you'll see much work is left to be done. So the IP Monitor will list the open connections on your system, for instance, but doesn't let you do anything with them (you can't manually close a particular connection, for example). And while most modules are easy enough to use - if you're familiar with firewall basics, anyway - their standard interfaces are currently basic in the very extreme.
This isn't a product for the security novice, then. FireBwall offers little in the way of handholding, and you're generally left on your own to figure out what's best for your system.
If you're knowledgeable about network security then this is unlikely to be an issue, though, and fireBwall may be worth a try just to get a feel for its future potential.