Backgammon, the game everyone's seen and at the same time nobody knows how to play :P

My brother and I once took a train trip from L.A. to Omaha and back for a friend’s wedding and played backgammon for most of the trip. For weeks afterwards, I saw backgammon everywhere (most notably when reading dialogue-heavy books with lots of 1-line paragraphs).

Solitaire and Hearts too. Well I actually know and love Hearts, but most people seem to know it as "that game in Windows where you play random cards"

I learned to play backgammon because it was one of the three games on my Nokia phone circa 2001 :P