For many years, the Fluid Dynamics Search Engine (FDSE) was $40 shareware. Supporters would receive a registration key that they could paste into their script. Those who paid were allowed to remove the publicly-visible copyright notice and link back to xav.com.
As of October 2015, FDSE is now free. Everyone is welcome to remove the publicly-viewable copyright notice and link back to xav.com. Nobody needs to buy or enter a registration key, but if you'd like to do so, a free universal key is:
I am very grateful for the years of generous support from so many users around the world.