Structural Simulation Toolkit was developed to explore innovations in highly concurrent systems where the ISA, microarchitecture, and memory interact with the programming model and communications system.
MacSim is a cycle-level, heterogeneous architecture simulator for x86 and NVIDIA PTX instructions.
gem5 a modular platform for computer-system architecture research.
DRAMSim2 is a cycle accurate model of a DRAM memory controller, the DRAM modules which comprise system storage, and the buses by which they communicate.
zsim a fast and scalable x86-64 multicore simulator.