Fiverr sounds like so much fun and I wish I had the time to do this. You simply place an advertisement for whatever product or service you have to offer for $5.00. It can be something simple, perhaps an advertising spot on your blog or website for a week. Maybe you do some sort of small craft or simply send a post card to the person from your city or state. Some people offer to advertise for others using their twitter account, doing a simple graphic design, placing a phone call to someone, the possibilities are endless, as long as they are legal.
From reading through the FAQs, the website does take $1 out of each fiver you make, but this could really add up if you have something to offer that a lot of people may want. You will get paid through paypal and your name will remain anonymous to the person you are selling the product or service to.
With a good imagination, I'm sure someone could make a decent amount of money. Give it a try and tell us what you think of if you are already using it, leave a comment and tell us how it is working for you!