Text Link Ads is a popular program for those that don't want graphic advertisements on their web site and want to be paid a fixed amount every month. This is also my best paying program for my blogs.
This program simply puts a set of links on your site in an area you specify. Easy as pie. You don't have to sell anything since the service sells the space for you. When you set up your site for the service you specify how many pages the links will be on and how many links should be shown on your site. You get paid based on popularity of your site and where the links are placed.
There are options to allow you to approve a link before it shows on your site, or to automatically approve any links. Typically people buying the links are looking to get a higher page ranking from Google as well as some traffic.
Text Link Ads are now coming out with new programs to show more links on specific posts of blog sites and have other programs ready to roll out in the future. While their main program will work on most any site or language some of the new programs require Wordpress so if you use other blogging systems you are out of luck.