hping

hping3 is a network tool able to send custom TCP/IP packets and to display target replies like ping do with ICMP replies.

hping3 can handle fragmentation, and almost arbitrary packet size and content, using the command line interface. Since version 3, hping implements scripting capabilties.

As a command line utility, hping is useful to test at many kind of networking devices like firewalls, routers, and so. It can be used as a traceroute alike program over all the supported protocols, firewalk usage, OS fingerprinting, port-scanner, TCP/IP stack auditing.

It's also really a good didactic tool to learn TCP/IP.