NetExpect's competitors:

Scapy
http://www.secdev.org/projects/scapy/

sendip
http://www.earth.li/projectpurple/progs/sendip.html

CASL
http://www.sockpuppet.org/tqbf/casl/

hping
www.hping.org
(hping version 3 embeds Tcl, just like NetExpect does.)

Nemesis
http://nemesis.sourceforge.net/

ISIC
http://www.packetfactory.net/Projects/ISIC/

Netwox
http://www.laurentconstantin.com/en/netw/netwox/

Mausezahn
http://www.perihel.at/sec/mz/

ostinato Packet/Traffic Generator and Analyzer 
http://code.google.com/p/ostinato/

dpkt
http://code.google.com/p/dpkt/

Bit-Twist
http://bittwist.sourceforge.net/index.html

T50
http://t50.sourceforge.net
