Data aggregation has been an important mechanism for achieving energy efflciency in WSN's. The aggregation reduces the transmission of redundant data which results in improved energy usage. A large number of data aggregation protocols have been developed in the past based on various techniques of optimizing the delay and energy. This paper surveyed the most prominent data aggregation protocols recently developed, from the categories of structured, structure free and hybrid. Based on the analysis, it came to light that the structured protocols fit well to the static environments (where the cluster head remains same and the nodes do not die), as the energy consumption can be kept at minimum by reducing the number of message exchanges required for setting up the network. On the other hand, for the dynamic environments, the structure free protocols are feasible as the cost of setting up a network structure increases. The hybrid data aggregation protocols have recently been introduced in order to take advantage from both the earlier approaches. The directions for future research are suggested in the light of surveyed protocols.