Circuit Simulation For Mac
I recently discovered that LTSpice is now available as a native Mac implementation.  Download it from Linear Technology for free.

It seems to work like the windows implementation although the menus are done differently.  It can use the same netlist files.

I have also used LTSpice under VirtualBox (free from Oracle) with no problems. You have to install windows in the VM with this approach.

LTSpice will run under Crossover on a Mac.  This does not require that you install Windows.  There were a few quirks with some of the dialog boxes but it was very usable.