Not the most dramatic magic item, but perhaps one of the most useful...
Stone of Concentration
A stone of concentration will enable a spellcaster as long as it is within his or her field of vision and/or touched to cast or memorize spells free from any distraction including injury. Only removal of the stone from the spellcaster's sight and touch will break his or her concentration. The spellcaster may willingly break concentration at any time. The stone may be used by anyone capable of perceiving it to aid in concentration.
Like other magic stones, a stone of concentration has no standard size, shape, or coloration. Unlike other magic stones, its properties can be discovered as soon as it is seen or touched by an intelligent creature.