You can use 'go get' to download and install packages direct from repository links, and there is a reasonable definitive list of open sourced libraries available here:

http://go-lang.cat-v.org/pure-go-libs

There's also the Go project dashboard: http://godashboard.appspot.com/project

Also gopkgdoc maintains a list of Go packages:

http://go.pkgdoc.org/-/index

The go command will install third party packages from source code repositories if you know about them. There is a dashboard of packages on the go web site but I believe further work is needed for discovery on the command line.