I really like the way the Calendar can be skinned. Recurring events are customizable, allowing me to place various promotion events periodically.
Support is really good and quick. They helped me set up an extension in minutes.
SQL to File has helped me a lot to provide lists of products as downloadable files for my customers
VJ Zoom is a really easy-to-use module that adds a fancy zoom effect to my images.
I found VJ Sequence Diagram to be an easy to use text-to-chart module.