There are many different types of such systems available on the internet. If you need it to get sign-ups for a website that will be launching, you can use http://launchrock.com. If you have a wordpress blog, you can use various wordpress plugins: http://wordpress.org/extend/plugins/tags/referral. There are many general rewards scripts you could use, e.g. http://www.hotscripts.com/listing/projektirs-referral-reward.... If you want to pay cash rewards, then you should use a general affiliate script. You can look at some examples here: http://www.hotscripts.com/category/scripts/php/scripts-progr...

What type of reward? If it's money, have you considered the regulatory costs of running it in-house -- the collecting W9s and cutting checks and mailing out 1099s and all that? That's the kind of stuff an affiliate network takes care of in exchange for a cut of the commission.

Its not monetary reward, but giving extra resource like dropbox provide extra space on referral.

Sounds like you're looking for curebit: http://www.curebit.com/

http://zferral.com helps you implement an affiliate program.