For a free GPL solution see the Snort inline fork - Suricata
Snort is going to be over taken by Suricata shortly - snort-inline has dies as a project.. (only the inline version is an IPS rather than IDS)
http://www.openinfosecfoundation.org/index.php/download-suricata
Suricata is 'more free' and as a result has funding by the USA navy (the creators of Tor) and the Dept of homeland security...