Text Link Ads is one of my favorites. You need some decent websites, but that is about all you need. When your website gets approved, you get a code that you have to implement in your website. From then on advertisers can buy links on your website for a set price. You can choose to manually approve each advertiser or leave it on automatic. Automatic works fine for me, but you have to check now and then, because sometimes some indecent stuff might slip in.. You can also choose how many links you want to show up on your site. They pay you prompt at the first of each month.