If you have an SSH server that is accessible from the internet then you should look at the DenyHosts application to protect your servers and networks.
DenyHosts protects your servers by parsing your ssh log for failed attempts at ssh login. The log where this is recorded varies by distribution. On Red Hat it is /var/log/secure and /var/log/auth.log on Mandrake. You should have one of these log files on your system
DenyHosts works by monitoring these logs for failed ssh login attempts. It also tracks which user accounts are targeted. When it finds a repeated failures from the same IP address it inserts these into your /etc/hosts.deny file effectively blocking the offending crackers.
Like any security measure this one can be shored up by implementing complementary measures. These would include disallowing root logins, using a port number other than 22 and disabling password logins. All of these can be set in your /etc/ssh/sshd_config file. Your ssh daemon must be restarted after making these changes.
You can download DenyHosts here.