* Commission Junction - They have a crazy amount of companies signed up that you can affiliate for. I originally signed up for eBay, but since then have been accepted into the NewEgg.com affiliate program and others. Many of the programs require you to signup individually for which program matches your website content, so you are not automatically accepted into their inventory of clients.
* Max Bounty - Generally speaking, they have the largest number of affiliate programs aimed at the simple 1 form, email gathering, affiliate programs. Meaning they have the $1 to $2 payouts for leading a visitor to submit their email for potential freebies and surveys.
* ebay Partner Network - They pay some of the highest commissions of any program I affiliate for. Their lowest tier of earnings makes you 50% of whatever eBay makes off an auction that ends with your affiliate cookie attached. Plus there are a lot of plugins and scripts to make building sites easy.
* Amazon - Who hasn't heard of Amazon? Some of their payouts are pretty low, but it's easily made up with the amount of volume that's possible. They also offer plenty of affiliate widgets, feeds, and store builder options.
* ClickBank - If it's a digital download, ebook, or some sort of online subscription service you can find it on ClickBank. They offer a huge inventory of these types of products that you can affiliate for with an even larger number of payout options. While I've only had limited success with this affiliate program, the potential is there if you can find a good niche.