It appears Craigslist, the very popular and old school directory listing site, has added the nofollow meta tag to most of their pages. If you view the source of the listing pages, you should see <meta name="robots" content="NOARCHIVE,NOFOLLOW"> in the header of the pages.
This tag was designed to tell a search engine not to follow any of the links on the page, including all the internal links. This is part of the reason the nofollow link attribute was designed, to give webmasters more control on which links should be followed by search engines and which ones should not be followed.
I find it interesting that Craigslist decided to simply nofollow all the links on the page, using the nofollow meta tag, as opposed to slapping on the nofollow attribute on user generated links.
As Google's help document explains, "originally, the nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow (i.e., crawl) any outgoing links on the page." But since the creation of the nofollow attribute value of the rel attribute, most sites have abandoned using the meta tag for the more controlled attribute.
Forum discussion at HighRankings Forum.