In our current campaign, our ninja decided to try to drop on a monster from a ledge 25 feet up, blade first. I house ruled that it would require an extra 10 to the Tumble of falling without damage, and the normal fall damage for the height would add to weapon damage. It worked well enough for our purposes, and the enemy dropped in one hit with the added sneak damage, so positioning wasn't a problem. Is there any official ruling for that situation though, in case he tries that again?
A character can damage someone else based on the rules for objects falling on creatures. For most characters, this won’t be much damage and even if you get it to count as a lot of damage, it’s not really an “attack” per se.
Aside from that, the only default rule is the +1 attack bonus you receive for attacking from high ground.
But with the right feats, you can fix that!
Cityscape has a short feat chain in Roofwalker and Roof-jumper, the latter of which gives you extra damage based on how far you have fallen on top of your enemy.
The Urban Class Features web enhancement for Cityscape has a Roof-dweller alternate class feature for the Barbarian, granting these two feats earlier than you usually could get them, instead of Fast Movement.
The Leap Attack feat from Complete Adventurer doubles your bonus damage from Power Attack based on horizontal movement. If you’re jumping from a height, you should be able to cover 10 feet horizontally.
Then Battle Jump from Unapproachable East is the best of them: if you drop from at least 5 feet above your opponent, your attack automatically counts as a Charge and you either deal double damage or count as a size category larger for the purposes of tripping or grappling.
Try to combine these with Pounce or with Tiger Claw maneuvers from Tome of Battle.
For more ways to optimize the art of jumping on your enemies for extra pain, I’ll point you to Little Red Raiding Hood: Guide to the Dragoon.
There's no official rule that I've ever found in the core rulebooks specifically for a PC jumping on an enemy (as opposed to any object falling on them).
When the situation came up in our game a while back, the DM ruled that if I made a successful Tumble check against the fall, the attack would be treated as a charge, which worked just fine for our purposes.
I would consider physical laws: A force acting on an object creates an equal opposing force. It is therefore an option to apply the falling damage to both the PC and the opponent. If the PC succesfully tumbles to avoid falling damage, he (likely) tumbles off the opponent and therefore doesn't apply extra force to the attack.
Perhaps a succesful tumble roll may allow the PC's damage to be converted to nonlethal damage.
Falling on someone is also a harder attack than usual combat, so I would apply a penalty to the attack roll.
I find it a shame if feats are required to perform such attacks. I would rather give my PC's some freedom to be creative - as long as it doesn't unbalance the game mechanics.
Falling on some has rules for objects falling on people (you add the weight of the object then modify the damage based on the distance to the target):
But the attack you ask about is truly preformed in the martial maneuvers of the Tome Of Battle (tiger style) You make jump check vs target number and kill your foe.