List products related to currently active category (url) My URL:
http://localhost/product-family/compressors

How can I list the related products? 
I don't understand how to get access to the compressors (a category entry of the product-family category type). The product-family is a structure - so there may be many levels. 

(I thought I'd have access to the currently selected category via entry but I get error when I try to access it.)
The same way as entry is available if you hit an entry, the variable category is available if you hit a category. 

Thus the following code: 

{% if category is defined %}
    {# Specific category selected #}             
{% else %}
    {# No specific category selected #}
{% endif %}