Invisible weapons, I've used before. You'd probably get to negate an opponent's dexterity bonus to AC plus get a +4 bonus from being an invisible attacker-but only on the first attack. Invisibility breaks on attacking, you know-and this does not change if you make it permanent.

A better deal would be to glammer a greatsword to look like a dagger. Less bonuses-but they'd work on every attack.