craigslist, backpage, gumtree, & kijiji. craigslist obviously has the most traffic by a long shot, but is the hardest to actually get posts live on. each has it's regional strengths. gumtree is primarily british, kijiji does well in canada. it varies from city to city, yadda yadda yadda. experiment with them and find what works for your particular method.
Craiglist as long as you don't spam everywhere and get your account deleted. You can google "Free advertising sites" to find some good ones.
Have you got project like posting on free classified sites? Wonder because I have seen these kind of job offers in news papers but most of them are useless or spam. But if you really want to use for advertising then it is good. Craigslist is the most popular one though you get plenty of them if you google it out.
Craiglist is very help full me. Also, I use ebay site in some country that allow to list classified for free