Craigslist is a website a bit behind in the times that fails to offer basic features to help users find things or avoid scams. Padmapper is a website that allows people to find listings from Craiglist (and other sites) on Google Maps. Recently, CL sent Padmapper a cease-and-desist letter to stop displaying CL results. Whenever a site becomes successful building off CL listings, they get sent this letter. CL as a website is quite poor, but they have a huge user base that makes it very difficult for a competitor to gain a foothold in the market. Both buyers and sellers just go to CL, since, despite its poor quality, everyone else is there. The easy way for companies to win is to display (a.k.a "steal") part of CL's listings so buyers will immediately find a useful website, and then they can get sellers later to list on their site. To prevent this from happening, CL needs to send out legal letters whenever a site starts becoming big. For example, in 2005, CL sent Oodle this letter.
The Cease-and-Desist letter normally results in the website complying, and that was what Padmapper's CEO initially said he would do. However, he just did an about-face and announced he would be using a possible loophole to continue providing the data. Instead of scraping the results from CL, he would use 3taps.com, which gets their results from Google's cache of CL. I don't know how that will play out legally, and it seems to be a shady move, but its potnetially good for the user. While CL is all "noble" about having free listings without even requiring an account or any identification, their site ends up being filled with scammers and spammers. If companies find a way to create successful alternatives to CL, the user will benefit.