You can start earning a handsome commission by refering products to your visitors. The bestsellers on web include ebooks, webhosting, electronics, games etc. You can also use the most efficient adsense referal program. There are many program available on net which are willing to pay you for referring their products(selling their products). Talking about the ebooks you can find thousands of them ready for sale. Many people too are willing to buy these even at high prices because they may find what they need and which is not available anywhere else on web. There may be free ebooks available for free but the ones from a well known and trusted authors are always preffered. You too can publish your own ebook if u think you are specialised in any field, you will just need is the ebook creating software.

Here is one :
Ebookcompiler

If you cant write one its alright you can refer ones written by others!
Here are some programmes which will pay you for referring ebooks on their sites :

Leisureaudiosales
Bizzydays-ebooks

There are thousands of people everyday planning to open their website. So this is an efficient way of referring a website package to users.
Here are some programs :
Sitesell
Thinkhost
Thirdspherehosting
Webwizards

There are many people searching for electronic products, and ofcourse they will prefer to buy it from you if you provide it at an affordable price. So advertise such product on your site and you can earn commisions from these. Here are some sites for you :

Computerstuff
Jhelelectronics

The next best thing, users are on the net for softwares and just softwares. You can earn serious money by promoting softwares. The fact that people search for free softwares is as true as there are people who are genuinely ready to pay for it.

Macecraft
Iphonegeek
2squared
Platinumpartner
Revenuegateway
Chitika