Since May 14th, 2019, it is now possible (from Brad Fitzpatrick)!

The #golang playground now supports third-party imports, pulling them in via https://proxy.golang.org/

Example: https://play.golang.org/p/eqEo7mqdS9l

Multi-file support & few other things up next.

Report bugs at golang/go issue 31944, or here on the tweeters.

(On the "multiple file" support, see, since May. 16th 2019, "Which packages may be imported in the go playground?": see an example here)

netbrain suggests in the comments another example:

On the playground:

package main

import (
    "fmt"

    "gonum.org/v1/gonum/mat"
)

func main() {
    v1 := mat.NewVecDense(4,[]float64{1,2,3,4})
    fmt.Println(mat.Dot(v1,v1))
}

woud give '30', using mat.NewVecDense() to create a column vector, and mat.Dot() to return the sum of the element-wise product of v1 and v1

The point being: gonum/mat is not part of the Go Standard Library.

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Original answers:

The most complete article on Go Playground remains "Inside the Go Playground", which mentions:

• godoc/static/static.go (for the default go program displayed in the playground)
• golang/tools/playground for the sources, with playground/socket/socket.go building and executing the program from the playground editor.

None of those processes support importing a remote package (that would be accessed over the internet).

It is very much a self-contained system (that you can run locally as well as using it from play.golang.org), with multiple features stubbed or faked, like the network:

Like the file system, the playground's network stack is an in-process fake implemented by the syscall package.

It permits playground projects to use the loopback interface (127.0.0.1).

Requests to other hosts will fail.

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Update 2017:

You have alternatives:

• iafan/goplayspace
• xiam/go-playground

But they still use use the official Go Playground service to build and run Go code, so that would still not allow for external imports.