Ay, ay, a scratch, a scratch. Marry, ’tis enough (3.1.94).
Mercutio's sense of bawdy humor is with him until his last moments. Although he has been fatally stabbed by Tybalt, he refers to his wound as a "scratch," yet also more somberly notes that it will be "enough" to end his life. Mercutio's ability to bring a naughty sense of humor to any situation makes him one of the most memorable minor characters in Shakespeare's plays.
Ask for me
tomorrow, and you shall find me a grave man (3.1.98–99).
Mercutio utilizes a pun here in the choice of the word "grave." On one hand, it reflects the seriousness of the situation (serious being an adjective not typically used to describe Mercutio); it also reflects that he knows that he will be dead by the next day, as he will be found in his physical grave as well. Mercutio's continued use of humor in the moments of his death reflect his ongoing wit.
A plague o’ both
your houses! (3.1.100–101)
Mercutio invokes a curse three times in his brief dying words. He is thereby shifting all of the blame to the Capulet and Montague feud, taking none of the responsibility for himself. Just lines before, Romeo tries to get everyone to calm down and not engage in this altercation. He stands in front of Mercutio, trying to protect him, and Tybalt stabs Mercutio under Romeo's outstretched arm. Mercutio chooses to engage in the fight, yet he curses Romeo and Juliet's futures and families for causing his death. The foreshadowing is heavy in this repeated curse, and the lovers themselves will suffer a quick end to their own lives. Interestingly, their deaths seem to remove the bitter hatred between the two families, so the curse on the "houses" doesn't seem to have the intended effect.
They have made worms’ meat of me.
Mercutio speaks gravely again, noting that they (and specifically, Romeo and Juliet) have caused his death; now worms will eat his remains. Mercutio likely enjoys a fairly high social standing as one of Romeo's friends, and he has fallen to the complete bottom of the world in his death and in this line. All is lost.