The Atronach Stone

Grants +50 Magicka and a 50% chance to absorb the magicka of incoming spells, but Magicka regenerates 50% slower.

Activating a standing stone grants its blessing permanently, but you can only have one stone's blessing active at a time — activating another simply replaces it, at no cost. The Warrior, Mage and Thief Guardian Stones near Riverwood are usually the first ones you reach.