As a DM, has anyone altered the Ranger’s Hunters Mark to make it a class ability, similar to a barbarian’s rage or Cleric’s channel divinity? Instead of using a spell slot to cast, they just get to use it once or twice per short rest, freeing up a used spell slot and giving them an extra spell in their arsenal. I feel that this would give rangers that little bit more playability and bring them up to a more similar level to the other classes. What are people’s opinions on this? Would it be too OP or is it something that others have done? 

Seems fair. I did something sort of similar on a warlock's Hex I played once, and it didn't really break anything. My DM dropped the Concentration requirements, and, once you get to level 5, it basically frees up a spell slot.

Seems only fair that Hunter's Mark on Rangers can do the same.

IIRC that's one of the changes that WotC was looking at for rangers with the class revamp UA.  I will try to look up the specific text.

EDIT:  Now with text

Favored Foe
1st-level ranger feature (replaces Favored Enemy)
You can call on your bond with nature to mark a creature as your favored enemy for a time: you know the hunter’s mark spell, and Wisdom is your spellcasting ability for it. You can use it a certain number of times without expending a spell slot and without requiring concentration—a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest.
When you gain the Spellcasting feature at 2nd level, hunter’s mark doesn’t count against the number of ranger spells you know.

That’s great. Thanks both for your help. Much appreciated. 

There is a lot more to add to to that if you just lookup UA class varients. 

op only asked about hunters mark