First what Fierce is not. Fierce is not an IP scanner, it is not a DDoS tool, it is not designed to scan the whole Internet or perform any un-targeted attacks. It is meant specifically to locate likely targets both inside and outside a corporate network.
Firewalk is an active reconnaissance network security tool that attempts to determine what layer 4 protocols a given IP forwarding device will pass. Firewalk works by sending out TCP or UDP packets with a TTL one greater than the targeted gateway.
Fragrouter is a network intrusion detection evasion toolkit. It implements most of the attacks described in the Secure Networks "Insertion, Evasion, and Denial of Service: Eluding Network Intrusion Detection" paper of January 1998.