This question should serve as a listing of all the different content-editable solutions. Post new ones in the answers below.
This is a community wiki answer. You can edit me with your improvements. Alphabetically sorted.
Inline editors differ from normal editors as they can edit the content directly, i.e. not placing it inside another element, or inside an iframe.
Iframe editors often load the editor or content to edit into an iframe, to avoid styling conflicts.