In Revolution, you can drap and drop any Element into the input areas, rich text or not, and it will provide a wizard that generates the tag and properties the user has selected in the UI.
Wordpress ha a very nice plugin for inserting snippets: http://wordpress.org/extend/plugins/post-snippets/screenshots/
If there is alrady such a thing in MODx please let me know. If not it would be great to have one especially for people who are not good at writing MODx snippets ([!SnippetName....!]).