Software
ns3 - http://www.nsnam.org/
GNS3 - http://www.gns3.com/
OMNeT++ - http://www.omnetpp.org/
Cisco Packet Tracer - https://www.netacad.com/it/web/about-us/cisco-packet-tracer (Proprietario)
Misc
mac80211_hwsim, a Linux kernel module that can be used to simulate arbitrary number of IEEE 802.11 radios for mac80211 - http://wireless.kernel.org/en/users/Drivers/mac80211_hwsim
Using CORE with the ns-3 Wifi model - https://code.google.com/p/coreemu/source/browse/trunk/daemon/ns3/examples/ns3wifi.py