eBay Classifieds, Backpage.com , etc Obviously it depends on how good your ad is, what city / section you post it in, etc. But would you generalize such sites as being worthwhile places for getting traffic to your blog or website?
It depends on what your site is. I manage a site for a car dealer and we post all of his vehicles on craigslist and he gets tons of traffic and sales from this. I've never tried it with another type of site but I think if you have a real product to sell it's good, otherwise I'm not sure. I know people at Microworkers pay to have ads posted on classifieds so they must be doing this for some reason!
I would definitely say they are a good place to get traffic. The key to it is in the keyword targeting just like anything else. Most people just use the classified sites for the built in traffic the sites contain..but the real gold comes in optimizing your ads so they may appear on the 1st page of Google. I've literally dominated the entire 1st page of Google multiple times before just by going to the most popular free classified sites and creating search engine optimized ads - WITH GRAPHICS..that's also very important because not enough people are doing that as well so it will make you stand out more. This works really good with offline marketing.