Local playground for the Go programs similar to play.golang.org. `M-x go-playground` and type you golang code then make&run it with `C-Return`. Playground works around `go-mode` and requires preconfigured environment for Go language. I recommend you to use `goimports` instead of `gofmt` for automatically make import clauses. It very comfortable especially for experimenting with code in playground. You may push code to play.golang.org with go-mode' function `go-play-buffer`.