What modern cultures practice paganism?

That depends on how you define Paganism.

If one wanted to "stretch" the definition, and culture that celebrates and or spiritually identifies with marking and celebrating the natural cycles of life and the earth.

In more general terms, many tribal and shamanic tribes in non-industrialized parts of the world still practice what we in the west would refer to as a paganism of some sort.

Some sects of what is referred to as Hinduism also constitute paganism.

Technically, the celebration of groundhog day and even Christmas is pagan in its origin.