All the major Pay-Per-Click search engines ie. Google Adwords, Yahoo, MSN's, Miva, Go Click, etc.
Here are the largest sites online to place free classified advertising.
http://classifieds.yahoo.com/ Alexa Rank: 1
http://classifieds.myspace.com Alexa Rank: 5
http://expo.live.com/ Alexa Rank: 8
http://www.craigslist.org/ Alexa Rank: 28
http://adlandpro.com/ Alexa Rank: 1,298
http://www.backpage.com Alexa Rank: 1,876
http://www.usfreeads.com Alexa Rank: 3,121
http://www.oodle.com/ Alexa Rank: 5,501
http://www.adpost.com/ Alexa Rank: 6,906
http://www.FreeAdvertisingForum.com 12,025
http://www.sell.com/ Alexa Rank: 13,471
http://www.InetGiant.com Alexa Rank: 21,082
http://www.adquest3d.com/ Alexa Rank: 22,252
http://www.recycler.com/ Alexa Rank: 23,996
http://www.chooseyouritem.com/ Alexa Rank: 19,569
http://www.domesticsale.com/ Alexa Rank: 24,207
http://epage.com/ Alexa Rank: 24,834
http://free-advertising-blog.com/index.php Alexa Rank: 26,048
http://www.ClassifiedsForFree.com Alexa Rank: 40,792
http://postaroo.com/ Alexa Rank: 43,471
http://www.TheFlyer.com Alexa Rank: 134,577
http://www.ClassifiedAds.com Alexa Rank: 143,204
Domain Name Coupons
http://webmasterautomation.com/blog/2007/01/21/domain-name-coupons/