NS2 (http://www.isi.edu/nsnam/ns/)
• discrete event simulator : Ns provides substantial support for simulation of TCP, routing, and multicast protocols over wired and wireless (local and satellite) networks
• Language used is a mixture of oTcl and C++
• need extension for WSN

Omnet++ (http://www.omnetpp.org/)
• strong GUI support and an embeddable simulation kernel
• Components (modules) are written in C++, then assembled into larger components and models using a high-level language (NED)
• Extension for WSN

Overall comparison of the simulation tools can be found in the following url: