Research from Cornell University, and published in this month's Science (requires subscription), indicates that there is purpose behind the mosquito's buzz besides keeping its human neighbours awake at night: male and female mosquitoes induce harmonic convergence within the frequency spectra of their hums as part of their mating practice. Contradicting earlier research which suggests that males have a highly limited hearing range whilst females are entirely deaf, both sexes were shown to modulate their buzzing frequencies to enter into harmonic love-making.

More info, and video, on Wired's Not Rocket Science.