There are several INET-based model frameworks, maintained by independent research groups:
OverSim is an open-source overlay and peer-to-peer network simulation framework for the OMNeT++ simulation environment. The simulator contains several models for structured (e.g. Chord, Kademlia, Pastry) and unstructured (e.g. GIA) P2P systems and overlay protocols.
Veins is an open source Inter-Vehicular Communication (IVC) simulation framework composed of an event-based network simulator and a road traffic microsimulation model.
SimuLTE is LTE user plane simulation model, with eNodeB and UE models.
INETMANET is fork of INET with experimental features related to mobile and ad-hoc networks.
MiXiM is not an INET extension, but an independent OMNeT++ modeling framework that focuses on the wireless physical, data link and networ layers. MiXiM can be used together with INET. Newer versions of INET (from 2.99.0 up) have the features of MiXiM, and much of the MiXiM code integrated.
The Vehicular Networks and Distributed Control Systems page lists extensions related to the automotive industry.
Several extensions are now part of INET (xMIPv6, HTTPTools, VoIPTools, etc.)
More INET extensions can be found in the OMNeT++ Model Catalog, such as ReaSE, HIPSim++, INET-HNRL, EPON, mCoA++, EBitSim, INET/Quagga, and others.