Currently there are a lot of distributions of Linux. The mainstream ones are these:
Additional information about the distributions can be found
here.
Security Enhanced Linux (SELinux)
This isn't a distribution per say, rather a set of tweaks and patches to improve security at the kernel level, but it is worth investigating.
Information can be found at
http://www.nsa.gov/selinux/
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got feedback?by howe81
edited by bmn 
last modified: 2004-11-13 03:02:45