The Network Simulator (NS-2)
NS-2 is the mostly used simulator for networks. It is designed though for wired networks and expanded later for wireless too. It consists of a simulator (NS) and a visualization tool (NAM).
The code can be downloaded from the homepage, support is given by the mailing list.
Some extensions exist especially for sensor networks. SensorSim is an extension to NS-2 from UCLA, which is unfortunately not supported any more and the distribution has been withdrawn.
The other extension for sensor networks, NRLSensorSim, is still supported and can be downloaded from the homepage. It adds a new type of nodes and channels into NS-2, the so called phenom. It simulates environment phonomena like carbon monooxide eruption or earth quakes and senses these phenomena through a designated channel, called the phenom-channel. The application on the sensor node is responsible to do something with the received data after that (e.g. flood the network). The current version of NRLSensorSim is for NS2.27 and cannot be used for later versions.
Note: My experience with NS-2 was pretty painful. Installing and running it (especially installing the sensor network extensions) lasted weeks, extending the simulator with new protocols is hard.