monomorph MD5-Monomorphic Shellcode Packer
This tool packs up to 4KB of compressed shellcode into an executable binary, near-instantly. The output file will always have the same MD5 hash: 3cebbe60d91ce760409bbe513593e401. Currently, only Linux x86-64 is supported. It would be trivial to port this technique to other platforms, although each version would end up with a different MD5.