All possible GOOS value?

I think you're looking for this list of possible GOOS and GOARCH combinations, in this section:

http://golang.org/doc/install/source#environment

$GOOS and $GOARCH The name of the target operating system and compilation architecture. These default to the values of $GOHOSTOS and $GOHOSTARCH respectively (described below).

Choices for $GOOS are darwin (Mac OS X 10.8 and above and iOS), dragonfly, freebsd, linux, netbsd, openbsd, plan9, solaris and windows. Choices for $GOARCH are amd64 (64-bit x86, the most mature port), 386 (32-bit x86), arm (32-bit ARM), arm64 (64-bit ARM), ppc64le (PowerPC 64-bit, little-endian), ppc64 (PowerPC 64-bit, big-endian), mips64le (MIPS 64-bit, little-endian), and mips64 (MIPS 64-bit, big-endian). mipsle (MIPS 32-bit, little-endian), and mips (MIPS 32-bit, big-endian).

The valid combinations of $GOOS and $GOARCH are:

$GOOS $GOARCH
android   arm
darwin    386
darwin    amd64
darwin    arm
darwin    arm64
dragonfly amd64
freebsd   386
freebsd   amd64
freebsd   arm
linux     386
linux     amd64
linux     arm
linux     arm64
linux     ppc64
linux     ppc64le
linux     mips
linux     mipsle
linux     mips64
linux     mips64le
netbsd    386
netbsd    amd64
netbsd    arm
openbsd   386
openbsd   amd64
openbsd   arm
plan9     386
plan9     amd64
solaris   amd64
windows   386
windows   amd64

You can see the list of supported platform by running:

go tool dist list

and this will print(depending on the Go version):

android/386
android/amd64
android/arm
android/arm64
darwin/386
darwin/amd64
darwin/arm
darwin/arm64
dragonfly/amd64
freebsd/386
freebsd/amd64
freebsd/arm
linux/386
linux/amd64
linux/arm
linux/arm64
linux/mips
linux/mips64
linux/mips64le
linux/mipsle
linux/ppc64
linux/ppc64le
linux/s390x
nacl/386
nacl/amd64p32
nacl/arm
netbsd/386
netbsd/amd64
netbsd/arm
openbsd/386
openbsd/amd64
openbsd/arm
plan9/386
plan9/amd64
plan9/arm
solaris/amd64
windows/386
windows/amd64

And the official documentation for the tool:

https://godoc.org/github.com/golang/go/src/cmd/dist

To cross compile use:

GOOS=darwin GOARCH=386 go build main.go

Note that those values are defined in:

• src/go/build/syslist.go, and
• doc/install/source#environment.

With Go 1.5 (Q3 2015), GOARCH will become much more complete.
See commit 1eebb91 by Minux Ma (minux)

go/build: reserve GOARCH values for all common architectures

Whenever we introduce a new GOARCH, older Go releases won't recognize them and this causes trouble for both our users and us (we need to add unnecessary build tags).

Go 1.5 has introduced three new GOARCHes so far: arm64 ppc64 ppc64le, we can take the time to introduce GOARCHes for all common architectures that Go might support in the future to avoid the problem.

const goosList = "android darwin dragonfly freebsd linux nacl \ 
  netbsd openbsd plan9 solaris windows "

const goarchList = "386 amd64 amd64p32 arm arm64 ppc64 ppc64le \
   mips mipsle mips64 mips64le mips64p32 mips64p32le \ # (new)
   ppc s390 s390x sparc sparc64 " # (new)

The list is still being review in Change 9644, with comments like:

I wouldn't bother with Itanium. It's basically a dead architecture.
Plus, it's so hard to write a compiler for it that I really can't see it happening except as a labor of love, and nobody loves the Itanium.

The official documentation now (GO 1.5+ Q3 2015) reflects that completed list.

---

Update 2018: as documented in Giorgos Oikonomou's answer, Go 1.7 (Q1 2016) has introduced the
go tool dist list command.
See commit c3ecded: it fixes issue 12270 opened in Q3 2015:

To easier write tooling for cross compiling it would be good to programmatically get the possible combinations of GOOS and GOARCH.

This was implemented in CL 19837

cmd/dist: introduce list subcommand to list all supported platforms

You can list in plain text, or in json:

> go tool dist list -json
[
        {
                "GOOS": "android",
                "GOARCH": "386",
                "CgoSupported": true
        },
        ...
]

As Mark Bates tweeted:

Bonus: Column output properly formatted for display:

go tool dist list | column -c 75 | column -t