Actually, most of the answers here are correct.
You don't have to use a real estate agent, or pay that commission, any more than you have to use a hair stylist to cut your hair, have an accountant do your taxes, defend yourself in count, or fill the cavities in your teeth. All of these things you can do on your own if you want to spend the time, and assume the risks involved.
Generally, 90% of people in the world are too busy to do everything themselves - and the time spent pulling comps, fielding calls, being present for showings, putting signs in their yard, advertising the property, negotiating, etc. takes away from the time they could be spending doing whatever it is THEY do and specialize in to make money. Regardless of the service, you will always have those who feel it's easy and want to do it themselves to save a buck. More power to them. Some people have nothing but time on their hands, are extremely strapped for money, and can't afford to have professionals doing everything for them.
If you don't want to use a traditional Realtor - I would suggest checking out a "flat-fee" MLS service. You'll be on the MLS, but get little or no direction/support through the process, and typically pay around $500 upfront regardless if you ultimately sell the property or not.