Link Reciprocation is quite a popular model for building link popularity for a website.
Karma and The Law of Reciprocation
Adding value. Karma. Give and you shall receive. No matter what you call it, it works … and works big time.
It all comes back to the fact that most people are generous by nature. If you give a no-obligation gift or do a good turn for someone, more often than not, they’ll feel compelled to repay you in kind.