Just recently I have been doing a lot of work on setting up and upgrading our internal and external networks. It has been quite an interesting and challenging job, that I am only just starting to get to grips with and I have learnt a lot about how to setup networks using open source software.
During my research, i have come across one particularly interesting project called PFSense. This BSD based firewall software is pretty robust, and provides a lot of the functionality you would get from proprietary, enterprise level firewalls.
What is even more interesting is that you can configure the software to suite applications other than just Firewalls (a list of examples can be found on the PFSense website), meaning that you can administer multiple parts of your network, using one common interface.
There is even a book, available from Amazon. Which is on its way to me now. Should be an interesting read.