There is no one solution that takes care of your network security. Protecting your services from the bad guys is quite complex and involved. Lets say we wanted to protect our openssh service for example; there are quite a few things that can be done in order to improve its security.
The first thing you would want to do is ensure that you have the latest version of openssh. You should also setup a mechanism that will check for updates regularly. Next is configuration. Server software can be quite complex to configure securly and openssh is no exception. Once your configs are all polished, there is still one attack vector that still could impact us, bruteforce attacks. Unfortunately, openssh doesn't have a feature that we can configure or enable that prevents bruteforce attacks. However there is a project, fail2ban, that can protect us from such attacks.
Fail2ban has been around for almost 5 years and simply works. It works by monitoring log files (in openssh's case, the auth.log file found on debian bases systems in /var/log/) for signs of failed login attempts. When it detects a number of failed attempts for a given duration of time, it can block the offending IP address using iptables and also send an email to the system admin. Fail2ban doesn't just work for openssh but can be configured to work for any service that logs fail logging attempts. See the links below for examples and details for configuring fail2ban for your services.
Resources / Good reading:
fail2ban.org
https://help.ubuntu.com/community/Fail2ban
http://www.howtoforge.com/fail2ban_debian_etch
http://www.the-art-of-web.com/system/fail2ban/
