I think at the moment your best bet is Google's Adsesne, as it is so far the top and reliable choice that there is. However with Yahoo's YPN and MSN's advertising system coming out soon, there will be more competition as each company will try to pay each person more to attract more advertisers for their sponsors.
as for myself, right now I'm using google's adsense, but I'm suppose to get an invite next week from yahoo's ypn. So far google's adsense has been good for my website, it's getting close to the point that I can just have it pay for my hosting and domain. However it is necessary for you, if you have a large site, to create channels for each ad so that you would no what part of your site the hits/money is coming from

. But like Marco said, the other site should be able to help you out even more.
good luck.