What do you mean by digital circuit simulator? The term digital circuit simulator is a bit broad...
1. Do you mean custom logic with MOS transistors generally used for extremely dense digital/high speed serial I/O?
2. Do you mean custom digital ASIC with standard cell libraries from TSMC?
3. Do you mean digital circuits as in FPGA / CPLD?
4. Or do you mean digital circuits as in actual digital chips on a pcb, and simulated with a SPICE like program?