Just a few notes on security on *nix systems.
While SSH is a relatively secure protocol, one of its' weaknesses is brute-force (or dictionary) password attacks. While all of my passwords are strong, I don't like the idea of someone trying to login to my system hundreds of times in a short period. DenyHosts is a Python script which runs as a daemon and prevents this type of attack. It continuously monitors failed SSH login attempts via syslog, discovers hosts which perform X failed logins in X seconds, and adds them to hosts.deny. It also supports whitelisting and blacklisting. Perhaps most interestingly, it has a group of over 6800 users which contribute their denied hosts to a central repository, which can then be sourced automatically by the daemon. This is a wonderful example of Global Blacklisting.
In the modern Internet age, many attackers set their sights on many different targets. One of the most obvious (and needed) solutions, therefore, is a way of distributing security blacklists between many different hosts. This can include lists of suspected attackers' IP addresses, spammers, and more. While many companies offer such services for a fee, it is obvious that the Internet needs a good, open-source solution to solve such problems.