For domain name registrars affiliate program, you can offer your customers value added service and domain related ads through the small banners or html codes on your websites. Generally you don't need the cost to become an affiliate. You can redeem your credits for cash or domain registration fee once you reachded a certain amount.

Here is list of affiliate program from the trustworthy domain registrars.


123-reg.co.uk:
http://www.123-reg.co.uk/affiliate.shtml

Domain.com:
http://www.domain.com/affiliate/

Dotster:
http://www.dotster.com/partners/

EuroDNS:
http://affiliate.eurodns.com/

GoDaddy:
http://www.godaddy.com/affiliates/affiliate-program.aspx

iDotz.Net:
http://www.idotz.net/ResellerProgram.cfm

Melbourne IT:
http://www.melbourneit.com.au/partner-solutions/affiliate

Moniker:
http://www.moniker.com/corporateservices/affiliates.jsp

Namecheap:
http://www.namecheap.com/affiliate/affiliate-program.aspx

NetFirms.com:
http://www.netfirms.com/affiliates/terms/

Network Solutions:
http://www.networksolutionsaffiliates.com/

Register.com:
http://www.register.com/affiliate.rcmx