Cragslist.org is the original site and can be pretty good source of leads, if you know how to do it right. It has many limits and rules that are not easy to go around. There are some other sites that could be used for advertising. They are probably not as effective as Craigslist, but also can be a useful source of leads.
Craigslist Alternatives list
It is very close with it's functionality to a Craigslist, it has a little bit smaller amount of people visiting it, and bigger amount of Spam and "Private offer" posts. It is still pretty big and powerful site that could be the biggest Cragslist alternative
2) Ebay Classifieds (Also known as Kijiji)
Ebay decided to do something with the competition (think Craigslist) and decided to get another similar site. US part of Kijiji was re-named to Ebay Classifieds
This one can be integrated with Facebook and also has some traffic and could help somewhere if you are trying to promote your product or service.
It can be used as Ecommerce platform
It is not that big, but this site has about 30000 visitors a month
8) BuySell Community
9) Gumtree (UK)
This one exists specifically for UK and is not available in other countries, but in UK this site is very important.
That is all for today.
If you know more or can get some tips about those sites, please make a comment.