There are now many Intrusion Detection (IDS) and Prevention (IPS) Systems, firewalls and log analysers out on the market. For my courses I've prepared an extensive classification of these systems, which at the same time works as a catalogue of available solutions (each is linked to its author or producer's website).
The tree is available in three formats - Freemind mindmap (source), SVG image and PDF document. I'll appreciate any corrections, e.g. of the categorization of specific systems or reports of new solutions available on the market. Main criterion I've used while categorizing these systems was its practical presence on the market. This is why it lacks some systems that were abandoned long time ago or exist only in development versions (e.g. hogwash or snort-inline).
Use FreeMind to view MM file.