There was a full house on the board queens over jacks. One player has pocket 9s and the other had a 9 and a 5... Who wins? Or is it a split pot?
Assuming this is Texas hold 'em, then the strongest hand each player has is the full house on the board so the pot would be split.
As per the article under The Showdown with relevant parts bolded.
On the showdown, each player plays the best poker hand they can make from the seven cards comprising their two-hole cards and the five community cards. A player may use both of their own two hole cards, only one, or none at all, to form their final five-card hand. If the five community cards form the player's best hand, then the player is said to be playing the board and can only hope to split the pot, because each other player can also use the same five cards to construct the same hand.
If the best hand is shared by more than one player, then the pot is split equally among them, with any extra chips going to the first players after the button in clockwise order.