If you mean using it with your own product, an affiliate program is the best option to improve your sales.
The aim of affiliate programs is to build a network of virtual sellers of your products. Using affiliate programs can help you get new sales through qualified and targeted traffic.
If you choose the revenue-share compensation model, you will pay only when there is an actual conversion, which may prove superior to traditional pay-per-click schemes where you pay even for non-converting traffic.
A big advantage of implementing your own affiliate program is that it'll expand your market. Instead of having an online store, it's like you get to have many more stores selling your products and/or services. In my opinion, it is worth a try.
It depends on what the particular case is. There are many companies willing to offer a ready-made affiliate program such as ClickBank, and there are competitors to them that are working very hard to earn the #1 spot in user preference, e.g.:
Truth be told, if you can get a nice custom affiliate program script programmed, you may do better in the long run since the ready-made solutions demand a cut of your earnings. But it really depends on your particular scenario. If you are the king of your niche, you can even turn happy customers into promoters rewarding them goodies and money. If you are a newcomer or are still relatively unknown, reaching webmasters willing to sell for you can be easier when you are a member of ClickBank or any other network.
Affiliate programs are great and if you can learn to drive traffic to your links, you can make some great income. I would never call affiliate income passive though, because you need to work hard to keep that traffic coming.