Debian OpenSSL Vulnerability
If you're running Debian or a Debian-derived Linux system such as Ubuntu, take note: a security vulnerability was introduced into OpenSSL in September, 2006.
Any keys you've generated since that time are potentially guessable.
For the inexperienced, here are the commands you should run at a bare minimum in order to fix it:apt-get update
apt-get install openssl libssl0.9.8
apt-get install openssh-server --reinstall
You should also destroy and recreate all your user keys if you don't know they were created either before September 2006 or on something that isn't Debian.
I'm probably leaving stuff out, but I'm sure the more experienced of the people in this community will correct me.