There are open source IDS products out there. If you need a Host IDS, or HIDS, then there is Daniel Cid's excellent OSSEC product. If you need a Network IDS, or NIDS, then the obvious choice is Snort.
While it was recently announced that Checkpoint would acquire Snort's owners, Sourcefire, it appears the application will remain open source and continue to be developed under a GPL license.
This was first published in December 2005