Logic Simulation/Testing Software - more Industrial Strength?

[Butters9000]

Is there any professional software packages similar to LogiSim but more powerfull?

I crafted RAM16K just like we did in chapter 3.  LogiSim got very slow.  I know I could have used the built in Memory chips but i wanted to roll my own.

What do the pros use to design more complex digital systems?

Anyone?