Wireless sensor networks have a wide range of applications because they can be adapted for various environments. They can operate independently in harsh places where a human presence is risky or even impossible. Since their life time is dependent on their batteries and replacing or recharging their batteries is impossible in rough places, it is necessary to find energy efficient routing protocols for them. In this paper, a number of well-know energy efficient routing algorithms for WSNs have been classified and presented based on their attributes.