Snort - the de facto standard for intrusion detection/prevention

2.2 Event Thresholding

You can use event thresholding to reduce the number of logged alerts for noisy rules. This can be tuned to significantly reduce false alarms, and it can also be used to write a newer breed of rules. Thresholding commands limit the number of times a particular event is logged during a specified time interval. See Section 3.8 for more information.