I was unable to find a custom html widget with Joomla style assignment. Like you create a module (widget), define some HTML and assign it to certain menu items (or set it to show everywhere except the selected etc). Given WordPress doesn't have the same menu system, the widget has to be able to assign itself to certain pages, post categories and tags. Hardcoding conditional tags into the template won't work...
Thanks for your help.
edited: I have come across this plugin. The problem is I need something easy to use for untechy guys. Like a set of checkboxes containing names of pages and categories.