Apologies for the cliché headline, but in this case, I think it's probably true; if you asked random strangers on the street what animal (in the widest meaning of the word) kills the most people each year, you'd probably get all kinds of answers like "sharks", "dogs", "snakes", "scorpions", but I don't think you'd get that many right answers.
In the #2 spot is of course humans, who are apparently pretty good at killing other humans with about a half million deaths per year. But the #1 spot is held with a pretty wide margin by mosquitoes, with around 725,000 deaths per year (and a lot more people being sick; around 200 million can be incapacitated for days at a time). As Bill Gates says, "Sharks kill fewer than a dozen people every year and in the U.S. they get a week dedicated to them on TV every year. Mosquitoes kill 50,000 times as many people, but if there’s a TV channel that features Mosquito Week, I haven’t heard about it."
The biggest killer is rather smallMosquitoes can be found in about 2,500 species everywhere on Earth except in Antarctica (it's a bit cold, you see...). Mosquito-borne diseases (malaria, dengue, etc) threatened about 3 billion people and cause many billion dollars of lost productivity each year, so any money spent on eradication of the disease is well-spent from a financial point of view, though even without that it would be a great deal from a humanitarian perspective.
The Bill and Melinda Gates Foundation has been working hard on the problem, and Bill Gate has been holding Mosquito Week on his blog this week to raise awareness further about the problem.
Via Bill Gates