The ein anderes Wort widget for wordpress higher than version 2.8 lists a certain amount of synonyms for a search term in the sidebar. The amount can be specified in the admin panel.
A button is added to the visual post editor which can be used to choose a synonym for the currently selected term.
ajax-based widget with search formular for the sidebar
the maximum amount of wordgroups to be displayed as and also the maximum amount of words out of each wordgroup to be displayed is adjustable
plugin for the visual editor for simple replacing terms with synonyms by a mouse click
multi widget ability which enables adding any desired number of instances of the widget to any number of sidebars