Best Network Intrusion Detection Software?

I currently have a linux box running Slackware, and it is my router. I have iptables setup and it is working flawlessly (I hope ). Now, what is the best intrusion detection software? I want it to log suspicious activity. And, if anything I would like a program that can log all of my iptables activity.

For intrusion detection, check out:
http://freshmeat.net/search/?q=intrusion+detection&section=projects&x=12&y=12
Logging iptables activity is a kernel function. Read the logging section of 'man iptalbes'.

edit: I'll be nice and post it for you

LOG
Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on all matching packets (like most IP header fields) via printk().
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up to 14 letters long, and useful for distinguishing messages in the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.

You maybe want to check out snort and nessus also.