The following were all located on the Kansas City Craigslist site on February 16, 2010.
The first posting is looking for a roommate who is a Christian

The next posting states an illegal preference for tenants with "no children"
(http://kansascity.craigslist.org/roo/2169633803.html).And this one excludes non-heterosexuals

The first two posts violate the federal Fair Housing Act. The third would be legal, except that the property is based in Kansas City, Missouri, which has a human rights ordinance that includes housing and protects on the basis of sexual orientation.
The purpose of this post is not to attack Craigslist. The site has a tool for reporting housing discrimination, and makes clear at least what the federal requirements are (although they do not explain, for example, that within the city of Kansas City, sexual orientation discrimination is also forbidden). The point is to illustrate that, with little effort, one can find discrimination occurring out in the open.