I joined 'cause I've been a Linux fan for a long time.
I started really using Linux (Red Hat 4.2 maybe?) in about '97, originally I used it for NAT, sharing one internet connection for my many computers on my local LAN. This was back before consumer routers were available.
Soon I started using IPChains, and later IPTables to do custom firewall rules, and allow for very controlled external access to my internal hosts.
At home I now use it for all of that, plus mail server, spam filter (MailScanner, SpamAssassin etc) web server, backups, tons of things. Also a great free open source video surveillance system to spy on my dogs while I'm at work.
At work, I use various security tools like Snort, Arpwatch, and network monitoring and a great helpdesk program. Some of these apps competes with the big multi-thousand dollar commercial apps for Free!