Great comedic monologues are hard to come by, so why not write your own? It's a great writing and acting exercise and it's also a fantastic resource for audition situations.
Create a flawed character: someone who is insecure, socially awkward, irrationally hotheaded or extreme in any way.
Imagine putting that character in a tense situation, such as a confrontation, a break-up, a confession of some sort or any other heightened, pressurized moment.
Become the character. Tape record yourself or take notes as you begin to try and get yourself out of a precarious situation.
Be sure to vary the moods or tactics-don't be one-note, such as angry the whole time. Let your character be manipulative or unpredictable with his or her reactions and emotions.
Imagine something unexpected happening in the middle of the character's monologue-something that is out of the character's control and changes the direction of the monologue.
Develop some recognizable physical or verbal characteristics/patterns throughout the monologue that become heightened until the end, when they either get in the way of the character or are dropped completely as the character reveals his or her true self.
End on a high-energy note, with the character more perplexed than ever, or self-deceived or confrontational.
Try and work in some physical humor, such as funny or awkward body language.