I have a requirement in which i have to show one meta data of my custom content model with html editor, so that we can set content in bold , italic and underline form.
You can create a new property for your type or aspect of data type d:content and a TinyMCE wysiwyg editor should show up for this control. Or for a data type like d:text configure a <a href="http://wiki.alfresco.com/wiki/Forms#richtext.ftl">richtext control</a> in Share .