FPDB is the oldest and for a long time was the only free hud, but it hasn't been updated since 2013.
https://sourceforge.net/projects/fpdb/
http://fpdb.wikidot.com/
The others that started free soon moved to monthly payment plans.
Most now just offer a free initial trial period.
The new kids on the block which are currently free are easypokerhud and pokermuck.
https://pierotofy.github.io/pokermuck/
https://easypokerhud.com/
I don't have any experience of them though, so it would be useful if anyone does try them, if they would post their experience or opinion.