SSH

Harden your SSH, consider these in your /etc/ssh/sshd_config file.

PubkeyAuthentication yes
PasswordAuthentication no
PermitRootLogin no
PermitEmptyPasswords no
AllowUsers alice bob carol
X11Forwarding no

Use keys, it's the 21st Century now. Also, you can tar-pit wankers trying to hammer their way in. This will drop anyone trying to connect more than three times in 30 seconds:

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 30 --hitcount 3 -j DROP