Which free VHDL simulator can I use?

Sigasi does not offer simulators. If you don't have a VHDL simulator yet, we would recommend the following simulators. You can download them free of charge.

Simulator Comment Download from Windows Linux Mac OSX
Aldec Lattice Edition Only Lattice FPGAs Lattice Download page Yes No No
Aldec Active-HDL, Student Edition Students only Aldec Download page Yes No No
ModelSim Altera Starter Edition Only Altera FPGAs Altera Quartus Web Edition Yes Yes No
ISim Only Xilinx FPGAs Xilinx Webpack Yes Yes No

At this time, Sigasi does not support Simili or GHDL. Note that GHDL on Mac OSX uses Wine, but works without problems.
We have not had good results with FreeHDL or with Green Mountain VHDL. It seems that these two projects may have been terminated.

You need a VHDL editor too

After you choose your simulator, you need a VHDL editor too. Sigasi has a free starter edition of its popular VHDL development environment.