Here are a few resources that will greatly help you in securing your operating system(s) and network for free!
Unified Compliance Framework's (UCF's) Security Technical Implementation Guides (STIG's) provide a down and dirty check-list style guide to securing a mulititude of operating systems and network devices.
Sarah Q., thanks for letting me know about that great resource.
The Center for Internet Security (CIS) has a more explanatory set of documents called CIS Benchmarks for securing many operating systems and network devices free for download. You will have to submit your email address to gain access to them, but I've never received SPAM from them, so I consider it safe.
I have a blog article on this already, but here's a free tool to help you assess security on and secure your *nix boxes called Lynis.
It's an audit tool for BSD, Linux, Unix users and Admins.
Go to town locking down your stuff!