Here is the list of legitimate Pay Per Click programs that allow you to advertise their own ads your website and earn a portion of what they make only if a visitor of your website clicks on a particular ad that caught their eyes and interest them. I've also added a little review of each of them, so you can get a better idea of their service. I recommend you to use Pay Per Click programs on your website if you want to make money. It's currently the easiest way to earn money online. The PPC programs are in no particular order.
The whole world knows about Google and its amazing Adsense program. This should be your first choice. The people at Google are very reasonable people you can deal with, so if you have question, you'll get an answer within 48 hours. The value of each ad varies to one keyword to another. It's the most trusted and the best PPC program out there; just make sure you respect their policies by reading them carefully. They only pay you by check.
Clicksor is a pretty good PPC program. However they might not always have the high click pay. It also seems that it slow downs the website for some reason. So it might not be as good as Google Adsense, but it's nonetheless a very reasonable service. You may be happy to learn that they do pay you via Pay Pal.
Yup, Yahoo too, has its own PPC program. However, only for the time being, only US members can join in. I heard that their PPC rates are higher than Google's. It's definitely a PPC program to watch out for in the near future, but it's very similar to Google Adsense, if you look at their ads.
This isn't the best PPC program around, but they do pay. The bad news is that their PPC rates are quite low (example, 0.10$ per click). The minimum payout is 10$, which is really not bad because they also pay you via Paypa. Also, the ads don't display themselves automatically; you have to choose a category of ads. But that's no big deal. However, I've heard that there aren't many publishers there.
I've heard a lot of good reports about Chitka. Apparently, the PPC rates are quite good. Maybe as good as Google Adsense. Their payout is 10$ and they too, pay you via Pay pal. I have yet to hear negative comments about Chitka. This should be an alternative to other PPC programs.
The good news about Adbrite is that there is no minimum payout. You can set your payout anywhere from 1$ to 100$. But unfortunetly (for some), they only pay by check. Many big companies use Adbrite and I haven't heard anything bad about them, except that they have an "average" PPC rate.
Finally, these are the main popular PPC programs on the internet. There are in total 28 PPC programs out there, except many of them aren't reliable. This is why I've only recommend you to choose one of these.
Be sure to read exactly what they offer and their policies at their website before signing up.