I cannot recommend Digital enough. It is quite new. Clearly inspired by Logisim but not derived from it. It has support for switches like FETs and relays, an

I cannot recommend Digital enough. It is quite new. Clearly inspired by Logisim but not derived from it. It has support for switches like FETs and relays, and you can implement open collector logic with pull-up resistors. It can analyze and reduce combinatorial logic. It can generate Verilog and VHDL output from a diagram. And, IMO best of all, you can implement individual components in Verilog/VHDL and plop them as graphical elements in your circuit; when you simulate, it will fire up iverilog or ghdl in the background and communicate with them transparently. It's fast too; one of the demo circuits is a complete processor and it simulates it at around 300 kHz on my machine.