It looks like Craigslist has severe server issues. The site has been down partially for most of the day. It's possible to call up http://newyork.craigslist.org/, but it is impossible to post an ad or call up the original website, http://craigslist.org, hosted in the San Francisco Bay Area.

I'm surprised a site of this size and relevance is running into server scalability problems.

Update:It looks like they are aware of the problem and working out maintenance issues. You can now access some sites of Craigslist, but not post an ad.