Here's a pair of articles about how great the free and open source linux operating system is for building custom appliances! I have experience building and implementing several of the appliance technologies mentioned in these articles, so get in touch if you'd like to discuss your business' needs!
Internetnews.com, "Vyatta 5 Advances Linux Routing": http://www.internetnews.com/infra/article.php/3809401/Vyatta+5+Advances+Linux+Routing.htm
TechWorld, "Linux ideal for software appliances, says Novell": http://www.techworld.com/opsys/news/index.cfm?newsid=111467&pagtype=samechan