Go is under more or less continuous development (although the pace and nature of changes is different near releases). The Go website, Go playgroup, and other resources are what you want if you're interested in the latest released version of Go, as most people are, but there are also some resources if you want to look at the latest development version, what is generally called the tip.

The official source code is at go.googlesource.com. Typically you'll want to look at the tree view of the main branch. There's also the Github mirror of Go, which is where the issues are and which may be more convenient to navigate. Getting your own local copy is straightforward, as is building Go from source.

Tip.golang.org is more or less what it sounds like. Generally I'll want the Go documentation, especially the Go language specification. Tip.golang.org has a link for the latest standard library documentation, which goes to pkg.go.dev/std@master. You can also directly look at the specification from your local source tree, in doc/go_spec.html, but it probably won't have formatting that's as nice. At the moment, godoc can be used to run a local web server to view the standard library documentation for a Go source tree (or perhaps only the source tree that it was built from, in which case you'll want to build the latest Go development version yourself).

(You can also use pkg.go.dev to get access to all tagged versions of the Go standard library documentation, which includes betas and release candidates as well as actual released versions.)

Famously and usefully, Go has the online Go playground. As I write this there are two ways to get a playground with the Go tip version. First, you can pick 'Go dev branch' from the version dropdown on the normal playground. Second, you can use gotipplay.golang.org. I believe the two are functionally equivalent, but the latter specifically tells you what development version it's using and also runs 'go vet' on your code as part of submission.

(The normal playground will also let you use the two currently supported Go versions to try things with, which is currently Go 1.18 and Go 1.19.)

If you want to look at the generated assembly code for something, the Godbolt Compiler Explorer is what you want. There are two ways to get the Go version; you can select 'Go' from the language dropdown on the main page, or go straight to go.godbolt.org. To get the development version of Go you need to select eg 'amd64 gc (tip)'; 'gc' is what the Compiler Explorer calls the usual Go toolchain, as opposed to gccgo.

If you want to use, try, or test with the latest Go development version, you may be interested in the gotip command. An interesting feature of gotip that's not available by just cloning the source repository and building locally is that it can build Go with a specific CL (what Go calls its pending changes). This may be useful if a Go bug report says that a specific CL may fix an issue you're seeing; you can (in theory) use gotip to build that CL and then use it to try your code.

I believe that the Go team is in the process of moving away from golang.org in favour of go.dev, so at some point the golang.org URLs here may stop working. Hopefully there will be go.dev equivalents of them, ideally with redirections from eg tip.golang.org to the new go.dev version.

(This is the kind of thing I write down for myself so I can find it again later.)