Weird, I made something exactly like this about a year ago [1]. I think theirs is more complete by now. I also made a package.json generator to go along with it [2], though that is more specifically for front-end javascript packages, so it doesn't have all the fields.
Just passing through. Anyways: Nodejitsu made package.json.jit.su well over a year ago (August-ish 2011 iirc) and I'm honestly a bit surprised to see it popping up here now of all times.
Generator's not bad. Nodejitsu never made anything like that on the web, though `npm init` and jitsu's built-in prompts fulfill a similar need on the cli.
Wow, that timing is eerie. I just checked the logs and the jspkg sample package.json is from July '11 and the generator is from December '11. Let summer 2011 become known henceforth as The Summer of Package.json.
[1] http://jspkg.com/package_json
[2] http://jspkg.com/generator