What is not very well known? Based on the numbers of downloads I am surprised that the Email module with templates is not used more often. Most of the apps sends emails. And with this one the mail messages can be made much more personal.
Some other modules I really like are the ones where you can adjust settings in the app instead of in the modeller like the Microflow Scheduler from Erwin 't Hoen. Otherwise every time the client wants to make an adjustment I have to create a new MPK. Now I can just adjust it in the app.
Although most people probably already know this widget, I don't think that many realise that you can use it to render html elements that you create in a microflow. This can sometimes save you from the hassle of building custom widgets.
The GroupBoxHelperWidget makes any GroupBox much more powerful and useful. With the helper widget, you can create dynamic captions for the boxes. All of a sudden, you've got much more possible ways to use the functionality.
I am personally a very big fan of the signature widget. But I am not sure if the users all apreciate the option draw something here to relax while we make some complex calculations...
Furthermore the simple chart widget is one I enjoy using to help get large amounts of data in datagrids visualised.
Survey Builder provides comprehensive features for building surveys and it's structured in a decent way, making it quite extensible. It's for very specific purposes though. I also like the Reference Set/Tagger.
Not really unknown, but you can do some really cool stuff with it. A good addition to the widget would be the ability to use templating. (The feature that has been added to the default Mendix text fields aswell)
The widgets Input Reference Selector and OnChange Inputbox are perfect for those data entry tasks where the user is familiar with the data.