Eye of the Beast is a rare trinket that is given as a quest reward. It gives a very large bonus to spell crit, and is a viable trinket well into the 60's.
This item is a quest reward from the Alliance questwhich starts with Marshal Maxwell at Morgan's Vigil in Burning Steppes. To complete the quest, you must kill General Drakkisath in Blackrock Spire and return to Marshal Maxwell.
It is also a quest reward from the Horde questwhich starts with Thrall in Orgrimmar in Durotar. To complete the quest, you must turn in the following item to Thrall:
The quest chain for the Horde begins in Kargath.
The quest chain begins by looting General Drakkisath's Command from Overlord Wyrmthalak in Lower Blackrock Spire.
Patch 1.3.0 (07-Mar-2005): Now correctly increases your spell critical hit chances.