Snort is an open source intrusion detection/prevention system (IDS/IPS) developed by Sourcefire. It is the most used IDS/IPS worldwide. Snort alone has no GUI to interact with, but it’s possible to connect several other Network Security Monitoring systems with it, like Snorby, BASE, OSSIM.
Sometimes it is difficult to set an existing IDS into a real system. Although snort is awesome i prefer using RHAPIS which is an IDS simulator http://rhapis-data.appspot.com