The Scavenger class is intended to provide deferential services, or less-than best-effort services, to certain applications. Applications assigned to this class have little or no contribution to the organizational objectives of the enterprise and are typically entertainment oriented in nature. These include peer-to-peer media-sharing applications (KaZaa, Morpheus, Groekster, Napster, iMesh, and so on), gaming applications (Doom, Quake, Unreal Tournament, and so on), and any entertainment video applications.

Assigning Scavenger traffic to minimal bandwidth queue forces it to be squelched to virtually nothing during periods of congestion, but it allows it to be available if bandwidth is not being used for business purposes, such as might occur during off-peak hours.

Here is a pretty good PDF on the subject of using it for mitigating DOS attacks...

http://cisco.com/application/pdf/en/us/guest/tech/tk759/c1482/cdccont_0900aecd80295ac7.pdf