There seems to be an infinite number of affiliate programs available on the internet.  Be sure to do you research before joining any of them.  It's amazing what you can discover with just a simple Google search.  When something is a scam, people are quick to post their negative experiences on the internet.  So, be sure to look for those bad experiences before deciding what programs to join.

With an affiliate program, you earn money through a commission.  Most affiliate programs offer banner, skyscraper, etc. type ads that have pictures and descriptions of their products.  If a visitor buys an item through the product link on your site, then you earn a commission for that sale.

An affiliate program can be a great alternative or even compliment to a PPC program. They can be huge money makers depending on the content of your site. Affiliate programs do not pay you when visitors click the ads; they only pay when someone purchases an item via the affiliate link. But, let's say you have a blog that reviews electronics and you have an affiliate link to purchase an LCD flatscreen TV. If someone purchases this high dollar item, depending on the program, you will make a commission of 4% to 20%. So, as you can see, there is huge money making potential with affiliate programs, if the content of your site lends itself to have visitors who would be interested in purchasing some high ticket items.

eBay

eBay offers an affiliate program. I have signed up for it and been trying it out. I must admit, it's a little complicated and difficult to use. If you are really good with all of the technical terms involved in e-commerce and marketing programs, then eBay might be a great option for you. I will most likely close my account because I don't like the way it's set up.

You can make money by becoming an eBay affiliate via the traditional route of placing live eBay auctions on your site, using your affiliate link.  If someone buys the product through your link, you earn a percentage of that sale.

What makes becoming and eBay affiliate interesting, is you can be an affiliate and make money without having a website or blog.  As an eBay affiliate, you can ask for an affiliate link to any auction and this link can be emailed.  This opens up an entirely different money making opportunity.  Here is how it works:

1.  Go to Craigslist and browse through the want ads.
2. Find an item that someone is looking for that you think you will be able to find on eBay.
3. Find an eBay auction that matches what the person on Craigslist is looking for.
4. Create your affiliate link to this auction.
5. Send a brief, friendly email to the person on Craigslist informing them that you found the item they are looking for on eBay and provide them a link.

If the person buys the item from eBay using your link, then you will earn a commission from the sale.  This tactic works best when you match up a local buyer with a local seller so that there is no shipping expense involved.  Also, if there is no shipping you can try to sale big ticket items like a car or a boat for a larger profit.

It might sound complicated at first, but all you're doing is matching up buyers with sellers.

It is free to become an eBay affiliated. You can sign up at this link.

Amazon

Amazon is another trusted giant in the industry that offers an affiliate program, which they call Amazon Associates. I have also created an account with Amazon and really find it to be user friendly. So far, I do not run a site that really has visitors that are looking to purchase items, so I have not been earning well with Amazon. But, I love the way their site works and how easy it is to create and edit their ads. In addition to ease of use, I think their ads look really good and you can customize the colors to match your site.  Amazon offers a great payment plan compared to other programs.  They do have a payment minimum of $100 for mailed checks.  But, if you choose to be payed via direct deposit or with Amazon gift cards, the minimum payout is only $10.  You can create an Amazon associate account for free here.

Widget Bucks

Widget Bucks is a refreshingly new approach to affiliate and advertising programs.  With Widget Bucks, you place widget ads for products on your site (almost just like Amazon's Associate program) and if someone purchases an item from one of these links, you earn a commission. 

Two things make Widget Bucks different.  First, Widget Bucks boasts a PPC ad widget that earns $3-6 CPM.  This might be the best payout on the entire market.  Second, they have a referral program.  If someone signs up for Widget Bucks via your link, you make 10% of whatever they make for an entire year.

Widget Bucks is safe, free, and easy to join.  You can try it out by clicking on the link below and signing up for your free account.

Chitika

Chitika is a really unique affiliate program. They offer linked purchases for commission, as well as PPC advertising, and a whole lot more. I have not tried them out yet, but will soon be giving them a try. I have read a lot of great things about Chitika. Many say they are second only to AdSense. You can sign up with Chitika here.