It depends what you want to simulate or modelling:
Digital:
SystemC, VerilogC
C/C++
VHDL/Verilog (Modelsim, LDV)
Analog:
Matlab (for very complex systems like receiver with rf frontend, sigma-delta converter, baseband processor)
C/C++ complex and difficult to read for other designers, but very fast
AHDL/VHDL-ams
Good for low complexity of the analog part, good for integration with digital system
Saber
Mixed Signal
a combination of the tools described above, in most cases slow due ot the performance loos of the simulator[/b]