Frampt's overly excited reaction should the player deliver the Lordvessel to him:
Heavens! You have done it! You have retrieved the Lordvessel! After a thousand years! It is you, it is really you! (excited roaring/cheering)(long, awkward pause) Er, heh. Forgive me, I really should calm down.
When Frampt falls asleep again the player is unable to use their service. You wake him up by hitting him once. Use of one's fist is best for maximum hilarity at the sight.
Patches will sometimes utter this gem when the player browses his wares:
Welcome to Trusty Patches' Trove of Treasures. We chop prices, not limbs!
All of the excuses Patches gives for attempting to murder the player are played for laughs as well. From claiming that he accidentally fell on a lever to trying to convince the player that he's their friend, he finds any way he can to plead for his life. The funniest is after he tricks them in the Tomb of Giants. He straight out tells them that he's a murderer and a deciever when he thinks they won't survive, but when they return he denies it and shrugs it off as an impulse he can't control.
The Channeler enemy's dance when they buff up nearby enemies. Getting their staff allows you to also do the dance, leading to several instances of people invading other player's worlds and merely standing there, repeatedly doing the dance.
To top that: the only game where player-versus-player matches which are usually serious business, often turning into a free-for-alls, instead becoming impromptu dance sessions.
Siegmeyer of Catarina repeatedly showing up in the most improbable places, often asleep, and, as he puts it after you wake him up, in "quite a pickle."
He falls asleep in Blighttown! Land of disease, toxic water, and giant mosquitos! And he did it standing up!
Some of the armour combinations can be this. For example, try wearing Smough's set of armour except for the helmet (even funnier if you customise your character to have a smaller-than-average head). Or wearing Smough's helmet, armour and gauntlets... combined with a Hollow Warrior waistcloth.
The Symbol of Avarice, a rare drop from Mimics that boosts item drop rate and increases the number of souls you obtain, at the cost of constantly draining your life. It's also...well, a description would do it injustice.
The description of the Plank Shield.
Provides minimal protection, but at a cost of moderate humiliation.
The Chameleon Sorcery enables the player to transform themselves into an inanimate object, such as a large urn or a shrub for example, to blend in with their surroundings. While this spell is active, it can be highly amusing to wander up to an NPC, initiate a conversation with them, and watch as said NPC appears to be happily chatting away to an urn. The sheer act of moving around while transformed is also hilarious to behold.
The description of the Rubbish.
Who in their right mind would bother carrying this around? Perhaps you need help.
Backstabs also briefly knock down the target. If done correctly, with the right environment, you can stab your foe in the back, leap behind a corner or down the stairs... and prepare to backstab them again once they've gotten up.
Similarly, an unarmed parry, which has the exact same effect as a shielded parry (no damage, window in the enemy attack for a riposte) except... with your bare hand. That's either this or a Crowning Moment of Awesome.
Sometimes, walking away from a character while in the middle of business will prompt them to say some funny remarks.
A darkly humorous example: If you kill off Smough first during the Ornstein and Smough fight, Ornstein walks over to Smough's body, puts his hand on it, and absorbs Smough that way. If you kill off Ornstein, however...Smough smashes him with his hammer, and absorbs him that way. What makes it better is that if you watch closely, Ornstein's finger twitches right before he's hit. He's still alive, Smough just doesn't give a damn.
At the entrance to New Londo, you find a bunch of non-hostile Undead who are on the very edge of going Hollow. Most of them are either crying or going insane... except one on the edge of the cliff, who is reclining in a "sexy" pose.