You can start with:

Cooja Contiki: http://www.contiki-os.org/start.html

NetSim: https://www.tetcos.com/

OMNeT++: http://omnetpp.org/

The simplest and most limited - Cisco Packet Tracer 7: https://www.netacad.com/maint.html

Also, see the following web site: https://instabug.com/blog/top-iot-testing-tools/

OMNET ++, Netsim, QualNet are good options

I think this site is good for you http://www.cupcarbon.com/

Or

https://instabug.com/blog/top-iot-testing-tools/

There is NuvIoT IoT Simulator, it is a free simulator: https://www.microsoft.com/en-us/p/iot-simulator/9nwh3926zt2r?activetab=pivot:overviewtab

Contiki and netsim are also free and useful for simulation of IoT platforms

Amazon is also offering an IoT devices simulation, here is an overview of the tool

https://aws.amazon.com/solutions/implementations/iot-device-simulator/?nc1=h_ls

Netsim is not open source software..if you have any link regarding its free usage please share...

We use NetSim extensively; large community of users and good support from developers.

Try NS3

I suggest Cooja with Contiki OS for IoT simulation.

Thanks @Yashar i have tried cooja .. but it has limited guide and tutorial...can you suggest any help

Unfortunately, the open source tools have many difficulties which one starts seeing as one progresses inside the tool. Without any support it is almost impossible to make progress on our own.

At our university we use a combination of NetSim and MATLAB.

Try to use this: https://www.youtube.com/watch?v=Qm-LdSY66PQ, but I think you also can use the simulator in link: https://github.com/ivanzy/SenSE-Sensor-Simulation-Environment

It depends on the network that you are going to simulate. There are different models for IoT networks in ns-3 as an open-source option. But for commercial simulators, you may try NetSim which has its own IoT libraries along with WSN.

MatLab along with Simulink can provide many functions for signal and image processing, optimization techniques, and machine learning in IoT. Many codes are available online and can be modified as per our requirements. Integration of codes from other software can also be done in MatLab.