GoDoc Playground (visit) 
The GoDoc Playground lets you make changes to your packages documentation and see, in (near) realtime, exactly how this will be rendered on godoc.org.
- Automagically fixes unresolved symbols so you don't need to paste in all your code, just a small snippet.
- Uses the actual godoc library to parse and render documentation so you can be confident your documentation will be rendered exactly as previewed.
- Completely in-browser: uses a GopherJS compiled parser/renderer so your code never leaves the browser. Even works offline!