A free mod for Witcher 3, by DJ_Kovrik.

If you’re a fan of The Witcher, you will definitely want to install this mod like everybody else! With this, the enemy health bar and name will be displayed at the top of the screen. This is a great way to keep track of the status of your combat opponents.

It also makes it a lot easier to fight multiple enemies at once because you can only see the health of the one you are currently fighting. It does not affect bosses and friendly NPCs.