1. Google Adsense. It is the leader of online ad industry currently. According to its description: “Because the ads are related to what your visitors are looking for on your site — or matched to the characteristics and interests of the visitors your content attracts — you'll finally have a way to both monetize and enhance your content pages.” If you have a lot of Asian traffic, this may be the best for you. Google pays you on net-30, monthly. (with minimum $100, if your account has less than $100, your balance will be rolled to next month).

2. Yahoo Publiser Network (YPN). It is still in beta testing and is open to US residents only currently. It was reported that the ads are less relevant compared with Google, but the value is slightly higher. If you have a valid US SSN or Tax ID number AND your traffic is targeted to US user bases, you may apply at: http://publisher.yahoo.com
List your refer link here: $50/year (non exclusive) or $150/year (exclusive)

3. Clicksor. Clicksor is controversial; some love and some hate. Usually, the ad from Clicksor may be not relevant to your contents, especially when your site has a lot of dynamic contents. One advantage of clicksor is that the minimum payment is $20; it is good for start-up. Another advantage is clicksor can pay you through paypal; this would eliminate check clearing time. Overall, the earning potential is lower than Adsense or YPN.

4. Adbrite. Adbrite is actually an ad market, where you can sell your ad space or buy ads. You can choose to show network ads when your ad space is not sold. Adbrite pays you on net-60 bases.

5. Etology. Etology is a new (in beta) player similar to Adbrite. If you have some unsold traffic, you may try it.

6. Paypopup. Paypopup is a reliable popup and popunder ad provider. Actually, it is in the same big family of Clicksor. You can choose popup or popunder; I would recommend popunder and once per session so that it will not affect your visitors.

7. Others. I know there are many others; but do you need so many? Please note, I omitted a few well known ad service providers because some of them are not friendly to free hosting services. So try the six reliable programs I mentioned above.