electronics simulator

Are there software programs that can run electronics simulation?

For example, if I build a logic circuit or transistor circuit, can some software predict voltage, current, frequency, etc at points in the circuit?

Any recommendations?

Thanks!

Ltspice.
It's a free SPICE simulator from Linear Technologies they use to showcase their switch mode power supply IC's but is a full featured general spice simulator. A little hard to get to learn, but very full featured. Runs under Windows and Wine under Linux.

I have been using the simulation software from Linear Technologies called SwCAD III to show how much horrible distortion a transistor makes without any negative feedback like this:

Switcher Cad III and LTSpice are just different names for the same piece of sofware audioguru.