ASIC or FPGA?
For ASIC, it will be difficult to find free tools. You have to go the
commercial route and/or be part of a university where such tools are
available. For FPGA, since you have tried Xilinx, try Altera Quartus.
Don't know much about FPGAs but I suppose the synthesis results are
targeted towards a particular device architecture (so its quite
different from ASIC).
All depends on what you want to do...Good luck.
Free tools from FPGA vendors are not "Open source". In the
end you want a tool that synthesizes to your hardware, so
the selection of vendor-specific tools comes down to
how you implement your hardware. I'm not sure why
you had trouble with Xilinx webpack. All of the other
FPGA vendors have free versions of their tools, some
with very little restrictions (e.g. Actel) others
restricting you to a subset of the devices they offer or
limiting your ability to simulate, etc.
For FPGA: Icarus has synthesis backends (use -tfpga option).
For ASIC:
vl2mv converts (a small subset of) Verilog to BLIF-MV
VIS converts BLIF-MV to BLIF
ABC synthesizes BLIF to a gate netlist
Philipp