DamageIndicators

Damage Indicators is a small mod that adds pop-up numbers showing damage done to entities and also indicators to show the mob name, type, potion effects, it's health, and a picture of it.

Config
Pressing period ('.') will open a simple config menu where you can change the GUI scale (default 75%) and toggle the portrait, skin (i.e. any texture pack GUI for the window), the showing of potion effects, and pop-off damage indicators.

By pressing 'advanced' you can adjust the position of the window on the screen and change its size for every mob individually.