Spade stands for Statistical Packet Anomaly Detection Engine. It is a Snort preprocessor plugin to report and score unusual, possibly suspicious, packets. The anomaly score that is assigned is based on the observed history of the network. The fewer times that a particular kind of packet has occurred in the past, the higher its anomaly score will be. Based on the SPICE Whitepaper.
You can download it from the following link: https://packetstormsecurity.com/files/download/23216/Spade-092200.1.tar.gz
Source: https://packetstormsecurity.com/files/23216/Spade-092200.1.tar.gz.html