Motivation: I wanted to have a new-tab page for my browser that show things that are actually useful for me like a few links, a weather forecast, best-HN and a notepad to dump some information.
So I built a website, which lets you edit its own code https://new-hacker-tab.com
How-To: In the bottom-right side is a button which opens an editor, to edit the page. The result is stored in the localStorage of the browser. I use that page every day and have different versions for work and spare time. If you messed up, just delete the localStorage of that domain.
Half-Baked: I would like to add sharing features, so when I have built a very nice widget/page, others can use it too, but so far the import/export functionality is all there is.