IDMEF/IDXP - http://www.ietf.org/ids.by.wg/idwg.html
libidxp - idxp.codefactory.se (although currently being updated)
libpcap - www.tcpdump.org
iptables/libipq - www.netfilter.org
snort - www.snort.org
libnet - http://www.packetfactory.net/libnet/
RFCs - rfc.net
rubicon - http://sourceforge.net/projects/rubicon/
ssldump - www.rtfm.com/ssldump/
hogwash - hogwash.sourceforge.net