1. Google AdSense: the most known and widespread contextual pay-per-click advertising program. I’ve been using AdSense since 2006, and I can say that they are trustful, and they pay as promised in their terms of service. You have to make at least $100 before they’d issue a payment.
2. Yahoo Publisher Network: in order to be accepted, you need to have a valid US Social Security or Tax ID number, and your website must be predominantly in English and targeted at a US user base. The payment threshold is also $100
3. Miva Monetization Center
4. AdSonar
5. Adify
6. AdBrite
7. ShoppingAds: not exactly a contextual ads service, but you have the possibility to choose keywords for your ads block, so it will display only ads related to whatever you choose. All Non-US traffic will always see CPA (Cost-Per-Action) ads, regardless of your campaign configuration. The US visitors will see Cost-Per-Click (CPC) ads.
This is an example of ShoppingAds ad (actually it’s a real ad, to show you how it looks) for the keywords “work from home”, “make money online” and another one which I forgot (if you forget the keywords, there’s no chance to see them again):
