This free for personal use firewall for Microsoft Windows ranks at the very top on leak-prevention tests (see here). It is much more powerful than the firewall that is included with Windows and can prevent more intrusions than other personal firewall applications.
Download at:
http://www.personalfirewall.comodo.com/
SSH clients are used to connect to SSH enabled computers. Many versions of Linux have a command line client built in and it is also included in OS X. To connect to a computer over SSH launch terminal and type:
ssh account_name@computer_ip_address
There are also some good, free graphical clients available:
Ubuntu: Built into the operating system. Go to Places –> Connect to server… then set service type to SSH.
OS X: Fugu
http://rsug.itd.umich.edu/software/fugu/
Windows: Putty
http://www.chiark.greenend.org.uk/~sgtatham/putty/
The SSH protocol can be used to securely connect to a remote computer. SSH access can be enabled in OS X by going to Preferences –> Sharing –> checking Remote Login On.
There are many client applications that can be used to connect to a computer running SSH (see here). When connecting to a computer over SSH a secure client application should ask to verify the key. The key is used to verify that the machine being connected to is the intended one. The OS X SSH key can be displayed by using the Terminal, which is located under Applications –> Utilities. After opening Terminal type:
ssh-keygen -l -f /etc/ssh_host_rsa_key
The key will be displayed as 16 hexadecimal characters separated by colons. If the string displayed by the SSH client does not match the one displayed in Terminal then it is likely there is a man-in-the-middle attack occurring and the connection should be terminated.