Admin widget on the Write Post page for inserting HTML encodings of special characters into the post.
Add an admin widget labeled "HTML Special Characters" that is present in the admin Add/Edit Post and Add/Edit Page pages. Clicking on any special character in the widget causes its character encoding to be inserted into the post body text field at the current cursor location (or at the end of the post if the cursor isn't located in the post body field). Hovering over any of the special characters causes a hover text box to appear that shows the HTML entity encoding for the character as well as the name of the character.
Note that when used in the visual editor mode the special character itself is added to the post body. Also note that the visual editor has its own special characters popup helper accessible via the advanced toolbar, which depending on your usage, may make this plugin unnecessary for you. In truth, the plugin is intended more for the non-visual (aka HTML) mode as that is the mode I (the plugin author) use.