Sourcegraph CEO here. The first extension is Codecov:
https://github.com/sourcegraph/sourcegraph-codecov#readme. It overlays test coverage info on code files on GitHub (with green/yellow/red line background colors). It also adds a command palette to GitHub (like the VS Code command palette) that exposes more actions related to viewing code coverage.
This extension API is in alpha. Feedback appreciated, especially hearing about what extensions you would want to use or build.