With affiliate programs, you add a specially-encoded link to your page advertising a site or product and get paid a commission on sales generated when someone clicks on that link and buys something.
Usually you get a check (or direct deposit) whenever your earnings exceed a minimum, usually $50 per month. Unpaid earnings carry forward without limit in most programs, until you finally accrue enough to meet the minimum for payment. Note that the minimum is per merchant, not per affiliate management system (like the ones featured below).
There are sites that run (or appear to run) their own affiliate programs, and those that work through one of a few companies that are in the business of managing affiliate programs for others.
Over time, I intend to provide links to all of the major players in the latter category, as well as to those sites that pay commissions to folks like me for helping them find new affiliates. (Meta-affiliates, what a concept!)
Launched in 1996, Associates is Amazon.com's affiliate marketing program. By linking to Amazon products and services you can add compelling content for your site visitors enjoyment and receive up to 10% in referral fees for doing so. Sadly, they don't pay for referrals to their affiliate program.
Commission Junction is a service bureau that manages affiliate programs for over 1,900 merchants. One of the top 3 affiliate sites I've found. They even pay $2 per affiliate signup and 5% of the referred affiliate's commission for each new affiliate your site brings to them.
LinkShare is a service bureau that manages affiliate programs for over 450 merchants. One of the top 3 affiliate sites I've found. They even pay $3 for each new affiliate your site brings to them.
Google AdSense is a fast and easy way for website publishers of all sizes to display relevant Google ads on their website's content pages and earn money. 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.
A shareware search engine submission service. Pays 10% of sales, monthly with a $25 minimum and an annual settlement. Goes 4 levels deep.