The best Classifieds Sites to purchase your Advertising from are those who are either on a large network (their classifieds are featured on many other sites) or those that have their own domain and put a lot of time, effort and money into promoting the site.

I have listed a few of these sites in the list below and I intend on looking into a few more that I could recommend.

1) Adandpro Classifieds.
Their classifieds are now featured on over 12,000 sites. They have a $5 per month Renewal Ad service. I have found this to be as good as, if not better, than Overture's Pay-Per-Click listings, which cost me betweeen $25 to $50 per month. With the AdlandPro's Renewal Ad, your ad is renewed every hour so it goes back up to the top of the page, it is in bold print and highlighted. Your ad won't be deleted from the database after the default expiration date, it's automatically renewed every month, unless you cancel. You also get a free bonus - Submission to nearly 400,000 directories free and 200 free impressions on top of their pages.
AdlandPro also has an affiliate program where you could earn a lot of money PLUS receive a wealth of online marketing advertising tools. Click here for information on the AdlandPro Gold program.
UPDATE: AdlandPro PPC! In addition to the classified system, which has successfully delivered traffic to AdlandPro's loyal advertisers for years, they have now added Pay Per Click results (short PPC). Advertisers are only charged when somebody clicks their link. Receive guaranteed "Real" visitors for as low as 1 cent per click!

2) U.S. Free Ads
This site has tens of thousands of members. For only $10 you can become a Gold Member for a full year and place unlimited classifieds. All ads you place appear before ads from free accounts. You can also upload photos to your ads with free photo hosting for your ad images, place and edit ads in real time, renew your ad(s) as many times as you like. U.S. Free Ads says: "Our site is 100% "search engine friendly" which means our entire site, including classified ads regularly gets spidered and included in search engines. Some of the ads on our system that are also listed in search engines receive thousands of views and hundreds of replies!"
FREE Trial Premium Membership at U.S. Free Ads!
One of the Top Online Classifieds. This is a High Traffic site. Their pages are quickly indexed by the Search Engines, giving your ads even more exposure. Check out all the benefits of a Premium Membership by trying the FREE Trial! It is currently available to all USFreeads Gold Members who upgrade to a premium membership. So I would suggest you get a Gold Membership, it's only $10 for a year. Then take them up on the Premium Membership Trial. As a Gold Member for many years, I can tell you that there are also many great benefits in that package.

3) Adpost.com FREE Classifieds With a database of over 500,000 members and more than 1,000,000 ads, you can be sure that you're onto one of the best... Their massive traffic translates directly to better results for you! In addition, as they are purely focused on advertising, visitors are highly receptive to advertisements. Therefore, your results will far surpass industry norms. Paid ads consistently generate click-through rates of more than 80%! ...and your advertising reach doesn't just end here... Your ads also get blasted to thousands of other classifieds sites! Multiply your responses wildly by having your ad appear all over the Internet.

4) Epage Classifieds Epage has been around for over 10 years and has build a huge network of over 27,000 member sites. They get 750,000 visitors each month.

5) EzClassifieds & Everyday Business Ads. EzClassifieds are on a large network. You have 3 choices for paid advertising:
A Basic Paid Ad - appears at the top of the regular listings, with options for enhancing ads: bolding, highlighting, adding icons.
A Premium Ad (Pay only for actual clicks!) - High impact ad links, visible in a distinctive red area at top of pages. Clicking on these links sends user to detailed classified listing (with optional links+free pictures).
A Sponsored Link (Pay only for actual clicks!) - High impact ad links, visible in a distinctive red area at top of pages. Clicking on these links sends user directly to your website (with no intermediate classified listing being show).