SiteEditor is the simplest MODX Front-End editor available. The integration is very straight forward and can be done by any front-end developer.
It is still in alpha, more features will be added in the future, so stay tuned. For now it supports the editing of any resource text field (including text TV's!). You can even integrate it so that you can edit menu button title's from the front-end. It also includes a rich-text editor for content fields.
Be sure to read the integration guide: http://rtfm.modx.com/display/ADDON/SiteEditor
New in SiteEditor 0.0.1-alpha1