What editor can you suggest to integrate with the backend of the web app I'm currently developing? I want to allow my trusted users to add articles that would be visible on the frontend. It should have some kind of markup language (to make basic customisation - lists, bold...) and if possible also the option to upload images.
look into restful_authentication (authentication), RedCloth (textile markup language), Hobo (admin interface), and paperclip (file uploads/attachments). You can piece together something with those.
If the editors of your site are not so technical, you might want to look into a Javascript markup editor instead, such as TinyMCE for editing articles - it looks similar to the gmail editor or the one here on S/O (bold / italics / etc), and you don't have to use textile markup.
You can also try routing to, say, admin/modelname, and create administrative controllers there. Hobo might be a way of doing it, but it is usually done manually.
Do you mean "editor" in the sense of a component you present to end users in the browser, embedded in the Web page?
Stackoverflow uses the WMD editor. It's good. I'm using it in a current project. It took me about 20 minutes to read the docs and set it up. It's fast, lightweight, works in most browsers, and is easy to use both for you and the end users.
It uses the "Markdown" markup language.