If you live in the USA, did you know that your tax dollars are being used for some really good purposes?
You better believe it. For example, the NSA provides some great guides and tools for securing your operating system, whether your are on a Mac, or running Windows, Linux, or Solaris.
Some of the guides can get a little complex (especially the Linux and Solaris ones), but even if you do some of what they suggest, you are increasing the security of your OS and are likely to learn a few things at the same time.
There are more resources from other parts of the government as well.