All of these are great. I don't think my group has nearly the quality of some of these. But I'll give it a shot:
Our more arrogant players will roll a 19, or 20 and instead of saying natural 19 or 20 they say "19 naturally" in a cocky voice.
When they do score a critical hit we use the optional damage table, like "throat cut" or "arm broken" and as the DM rolls the mantra "come on something good" is shouted usually.
I have a ranger named Lysander, well he is 10th level and his mainstay is killing giants. the DM put us in the First Giants module and being in character Lysander attacked every giant. needless to say he died several times (this was second edition before I crossed him over, he had a CON of 16 or 17...when the module was done it was 12). So, when it was his turn he was often reffered to as Dead Guy or mostly dead guy. Or Ly-dead-er.
There has been on occassion, farting to enhance spell casting (that is to say the player farts as they tell us what spell they are casting. works best with fireball), but invariably leads to stinking cloud jokes.
One player hates when the bad guy gets away or some NPC that really hassled them, so when this happens, he shakes his head and says, "their on the list." (And he does have a list).
As a DM, my players will often finish my statement of how a night without encounters, so I must use this often, "And the night passed uneventfully, as it were."
I'm sure i am missing some. But I hope those are fun for you all.
