Amidst the battle for Net Neutrality, there's some discussion happening around Cox Internet, who provides software to customers that intentionally blocks Craigslist. Why would they do that? Well, because Cox has its own classifieds service, of course.
The background: Whenever you sign up with just about any Internet service provider, they provide you with a bundle of software that they want you to install and use. Outside of AOL, no one actually has to install the stuff in order to use their internet service, but most people don't know that. Thus, people get the install CD, and happily plug away through (often crappy) software that the ISP bundles.
In the case of Cox, they provide a little suite of apps called the "Cox Security Suite." Any unwitting (meaning, non-geek level) Internet subscriber would probably want to feel secure online, so they go ahead and install it. The ugly part is when this so-called Security Suite also blocks universally-accepted-as-awesome websites like Craigslist.