Back in December, Riot Games Inc. announced their intentions to create a program to reward people that invite friends to join League of Legends. And this day they made it real: the Refer-A-Friend program was announced today with awesome rewards. In the words of Steve 'Pendragon' Mescon himself:

It has now come to that glorious time when League of Legends players the world over shall band together and tell all of their friends to come join them in combat on the Field of Justice! And why, you ask? Because we’re going to give you free stuff for doing so!
As we all know, League of Legends is more fun when you play it with your friends. And so, to encourage you to help your friends broaden their horizons, we’re going to give you rewards for getting them to sign up. Every time someone that you have referred reaches summoner level 10, you’ll be eligible for a reward. This reward can be anything from an IP boost, to a unique champion skin, to a free trip out to the Riot Office in sunny California to design your own champion.
Here’s how it works: every fifth friend that reaches level 10 will net you an IP boost that you can redeem whenever you want. In addition, every time you reach one of the following reward tiers, you will receive a unique reward.

Source: Refer a friend, get rewards thread.