Check out Sourcegraph: web source tree browsing with jump to def, find refs, and a bunch of other intelligent features. For example, try diving into the Go std lib: https://sourcegraph.com/github.com/golang/go/-/blob/src/net/...

(Disclaimer: I'm one of the creators. We currently only support Go officially, but other languages soon on the way via the open-source Language Server Protocol standard. Also, big fans of GitHub and glad to see this move!)