OpenSEC Picks
libnet - Libnet is a collection of routines to help with the construction and handling of network packets. It provides a portable framework for low-level network packet writing and handling. Libnet features portable packet creation interfaces at the IP layer and link layer, as well as a host of supplementary and c complementary functionality. Still in it's infancy however, the library is evolving quite a bit. Additional functionality and stability are added with each release.

Spak - a collection of tools that can be used to generate and/or send arbitrary packets to a socket. Spak is designed to be very modular and allow as much control over the design of the packet as possible.

More Tools
Ipsend - packet generator from the ipfilter package

Ippacket - a command line/curses utility to construct IP/TCP/UDP/ICMP packets on a Linux system. (site down)

NetPacket - a bundle of modules disassemble nehtwork packets into a Perl hash for various Internet protocols. There are hooks for assembly of packets, but they have not been implemented in this version. At present, decoding for the following protocols has been implemented: Ethernet, ARP, ICMP, IGMP, IP, UDP, TCP.

Socket Script- made for people that want to create networking-oriented programs, but don't want to learn all the socket stuff. It has multiple network commands that enable you to tell the script interpreter where you want to connect, and all you have to do is focus on the script itself, leaving the connection parts to SScript.

tcpreplay - a tool to replay saved tcpdump files at arbitrary speeds. Many NIDSs fare poorly when looking for attacks on heavily-loaded networks. Tcpreplay allows you to recreate real network traffic from a real network for use in testing. Tcpreplay is fairly portable, relying on libpcap and libnet for packet capture and raw IP packet construction.