Ctrl+Shift+I or F12
-> Elements, It should show your source. Right click on any element and click Edit as HTML
Edit:
There are some extensions that look like what you want: https://chrome.google.com/extensions/search?itemlang=&q=editor
ChromeEditor
& Live WebSite Editor
look promising.
Added by barlop
I see that (after ctrl-shift-I / F12), right clicking the <HTML..> or </HTML>
tag and clicking "edit as html". allows me to edit anywhere within it. Choosing the opening tag means the popup for editing just covers all the html source that's there - which is very neat in terms of space when editing. Choosing the closing tag might be useful as the popup then appears under the html so you see a before/after. Or better, as benjamin says, F2 to edit, ctrl-enter to commit.