When it comes to FPGA design directly onthe Mac, your options are limited. I am using JHDL fromBrigham Young University (http://www.jhdl.org/).It's a Java API that supports design with Xilinx FPGAs. It has some limitations but is very interesting.
In the past I have used the Xilinx Web Pack tools running under Virtual PC. You need toget a USB to Parallel port converter to use the Web Pack tools to actually program a device or load a demo board. I should note that I have not tried this since Microsoft acquired the Virtual PC product line! Your results may vary. I ended up purchasing a SunPCi card to program parts on my Ultra-5 when I switched my Macs to OS-X