After years of advertising, I say the best (and most cost effective alternative to Google is Buysellads.com.
AymanX, I understand. I know that I need to match my niche, but I guess such site has all of its sites categorized and there is no problem with that (buysellads do the classification). I just want to know where I can find similar sites...
It depends on the type of website you working on. My recommendation as buysellads alternative is adhitz. They are awesome for "money making website types" , you can select high traffic website only or website networks. The price for US trafficstart from 10 cents a click
You can certainly advertise on someone else's site, but make sure that it relates to what you are selling. By the way, that was a great list posted by AymanX.