Go to Gmail and click on the "Report a bug" link at the top of the page. You might see a completely different interface that looks just like the Google Feedback extension tested in March. The main difference is that it works in almost any browser and you don't have to install an extension to use it. (If you don't see the link, then this feature is not yet enabled in your account. Adding your email address to this page might help.)
After clicking on "Report a bug", Google shows this message: "Please scroll until the issue becomes visible. Next, click anywhere in the window to take a screenshot." Click in the window and Gmail loads a new page that lets you highlight the problem, black out personal information and describe your problem. It's a really cool feature which will probably be added to other Google services in the near future.