@nicolas search for theme exodus.its persona fork with widgets, it not finished but it can help you to see the code
Popular Tags Widget
Here is another widget, this one displays the popular tags in your forum. You can set how many tags to display in the widgets ACP.
Nice, How about a 3D tag cloud plugin?
(click on Create Sphere 3D)
Both suitable for different niches. Though I love second one, I will be choosing first as it is more suitable in my kind of forum.