For example, the Snort IDS is extremely popular. It's an open source network intrusion detection system that is widely used in the enterprise. As an open source product, Snort is available at no cost and has a large community of developers creating rules.
Sourcefire, the company behind Snort, makes an official ruleset available to Snort users either in real-time (for paid subscribers) or on a 30-day delay (at no charge). This is the best way to obtain a reliable, timely ruleset. If you're using a different IDS product, consult the vendor for details on rulebase subscriptions.
This was first published in June 2008