Today, Google Project Hosting team launched a new feature: the ability to edit your source code files directly in the browser, in our online editor powered by CodeMirror. Just look for the "edit file" link on files in the online source browser:
As you edit, you can preview the diff of your changes so you know exactly what you are committing:
And if you don't have commit privileges to the project? No problem. Instead of committing your changes, you can file your changes as a patch in the project's issue tracker.