What about a technologically advanced weapon like a Chain-Sword, everybody likes Chain-Swords? Or a Power-Axe, Lightsaber or something similar.
In my campaign, a very short-lived Warpriest of Oghma had a hammer/scepter that gave him a cumulative +5 to creature-lore roles whenever it hit a creature. So he gains knowledge about it every time he hits it. The usefulness of that really depends on how you handle those lore rolls in your game. In my game at the very high end of skill rolls he could tell how many HP the monster had, if it had recharged its powers and even what it was planning to do next turn.
What about a book that is basically a magical repository of combat methods? So the book is a holy item that lets you use a bunch of feats and special abilities each time you study it. So you kind of have say 3 linked feats or at-will powers that you can swap around during each extended rest. For instance: "The Way of the Avalanche" gives Bludgeoning Expertise, Battering Shield and the a shield-bash at-will.