I have been thinking about this a lot (you will find out more later ).
As usual it all depends on what you are trying to do. If you are worried about your connection to the Internet then consider a DSL/Cable Router/Firewall some new versions even have VPN so you can remote into your home network and have secure, trusted, Internet access (great for travel). If you are thinking a bit more robust then you might consider Gentoo http://www.gentoo.org/doc/en/home-router-howto.xml which is Linux or pfSense http://www.pfsense.com/index.php?id=17 which is BSD. With the Gentoo you will be doing a lot of command line configuring. pfSense has console configure and a web interface and a lot of great looking services. I have just started looking into deploying this but I hear it is great and easy to use.