I've never been too impressed with how curses on weapons are handled in D&D. A penalty to hit and damage instead of a bonus...yawn. There's no flavor or mystery to it, just basic subtraction. When I think of a curse, I think of a terrible price paid for some meaningful benefit, or a penance for the sins of a past wielder, not a penalty for just picking up a sword.
The standard sword -1 format need not be abandoned completely, but these curses are intended for weapons that have useful abilities, perhaps even powerful ones. In some cases, a character may bear the curse willingly to keep the weapon, particularly in a campaign in which magic weapons in general are scarce.
Justice: Whenever the weapon is used to kill a creature of good alignment, or one not guilty of any wrong, the wielder must save vs. Death Ray or be struck dead on the spot. Appropriate for a weapon once carried by a fallen paladin or knight.
Sorrow: Before the wielder rolls to hit on any attack that could kill the target (i.e. the target has fewer hp remaining than the weapon's maximum damage) he must save vs. Paralysis or be struck numb with remorse for his intended actions and unable to act for 1d4 rounds. Perhaps the weapon once belonged a warrior who regretted the lives he had taken during his career.
Taint of blasphemy: The weapon's bearer is subject to turning by clerics of Lawful or good disposition, and if he steps into a temple or other hallowed ground of a good faith, he must make a saving throw vs. Spells each round he remains there or suffer 1d3 points of damage. Pious people will sense something amiss as well, and may shun the character. This weapon may have been owned by a cleric who betrayed his faith in some heinous way.
Enemies' ire: When combat is joined, the most powerful adversary will make every effort to attack the wielder. In the case of encounters with a single enemy, it will always choose to target the weapon user if at all possible. The weapon was likely the property of someone who boasted once too often of his combat prowess.
Death's shadow: People are deeply uneasy in the presence of the weapon bearer for reasons they can't identify, and they will greatly desire to leave his presence as soon as possible. NPCs and monsters react with a -3 penalty. By contrast, undead creatures are drawn to the bearer as if he were a kindred soul, and beckon him to take his proper place among them. Very likely the weapon belonged to someone who cheated death or fate.
Rage: When combat begins, the wielder becomes enraged. When combat would normally end, whether by defeating all enemies, surrender, or flight, the wielder of the weapon must make a saving throw vs. Spells each round or continue to attack the closest target, friend, foe, or neutral. Perhaps the weapon belong to a wrathful or ill-tempered person who slew a friend in anger.
Burglar's bane: Whenever the weapon's bearer tries to sneak or move stealthily, he must save vs. Paralysis or the weapon betrays him by slipping from its sheath to clatter to the floor, or if held, causing a sudden tic of the hand to make the character drop it or strike some noise-producing surface. Rumor has it this weapon belonged to a thief who stole from his companions for many years before being caught, cursed, and banished from the group forever.
Mariner's curse: If the weapon bearer should fall into any body of water, the weapon drags him like a stone to the bottom, and he is unable to let go of it. Likely the bane of many an unlucky sailor.
Monstrous deformity: The bearer of the weapon begins to develop features of some type of monster - scaly skin, yellow eyes, warts, thickened and elongated nails, clumps of wiry body hair, etc., and perhaps some of its tastes and habits as well - an appetite for raw meat, aversion to sunlight, or some such. This might occur with a weapon that grants powers or abilities reminiscent of a particular monster type (e.g. a sword that allows the user to sense the presence of gold may make him resemble a dragon; one that drains energy levels might give him the deathly pallor of a wight) or one meant to combat a specific type or class of monsters. A character with a cursed sword +1, +2 vs. goblins might start to resemble a goblin in appearance and habits. ("He who fights with monsters might take care lest he thereby become a monster." -- Friedrich Nietzsche)
Nightmares: The bearer of the weapon is plagued with terrible nightmares, and each night must make a saving throw vs. Spells or awaken unrefreshed, healing no damage naturally and unable to memorize spells. If that throw is failed, a save vs. Death Ray must be made, with each failure resulting in a loss of one point of Wisdom. A successful night's sleep will restore 1d4 lost points. If the character's Wisdom drops below 3, he has lost his grip on reality and become insane. The insanity is permanent, unless treated with a Cureall spell or similar powerful healing magic.
Pestilence: The weapon attracts insects and vermin, which begin to infest the bearer's clothing and rations. He will eventually be infected with any number of disfiguring and debilitating diseases, but the weapon's curse prevents any of them from killing or disabling him - any debilities are token only, imposing no more than a -1 penalty to any action. NPCs are likely to act with revulsion and may refuse to deal with the character at all, and his diseases are communicable to others. A Cure Disease spell functions normally, but the sicknesses will return soon so long as the weapon remains in the character's possession.
Changeling: This weapon was made by fey enchanters, and never meant for mortal hands. When wielded by a mortal, the fey magic sometimes overwhelms him. On any natural roll of 1 or 20, an unexpected magical effect occurs. (A chart of such effects will need to be adopted or developed.) In all likelihood, the weapon was stolen from the faerie court by a mortal too clever for his own good; the faeries may be actively seeking it, which may add to the current bearer's troubles.
Spell magnet: The weapon draws magical energy to itself. Any spell cast at a target within five feet of the bearer arcs over to the weapon instead, and is conducted along it and into him. A normal saving throw applies to resist or reduce the effect. If the spell is cast directly at the bearer, the curse draws in so much of the magical energy that a -2 penalty applies to the saving throw.
Eternal peril: The weapon calls silently to monsters. While it is carried, the chances of random encounters double.
Power siphon: The weapon's bonus to hit and damage functions, but by drawing life energy from the wielder. On each successful hit, the wielder loses hit points equal to the weapon's bonus. A character attacking an enemy with a +2 sword does get a +2 bonus to hit, and if he hits does +2 damage, but takes 2 hit points of damage himself. No damage is incurred on a missed attack.