The Xilinx webpack is free for hobbyist level devices. You can implement a lot before you hit its resource limits. Also, in the past (don't know about now) you

The Xilinx webpack is free for hobbyist level devices. You can implement a lot before you hit its resource limits. Also, in the past (don't know about now) you could get a version of Modelsim with the Altera Quartus toolchain. It could be used with Xilinx simulation libraries as well, although that technically is a license violation.