You can do this with the OpenSSL command line tool:
openssl zlib -d < $IN > $OUT
Unfortunately, at least on Ubuntu, the zlib subcommand is disabled in the default build configuration (--no-zlib --no-zlib-dynamic), so you would need to compile openssl from source to use it. But it is enabled by default on Arch, for example.
Edit: Seems like the zlib command is no longer supported on Arch either. This answer might not be useful anymore :(