Due to many advantages over traditional networks, in recent years, wireless sensor networks have offered a spectrum of new applications. A middleware layer bridging the gap between applications and low-level constructs is a novel approach to resolve many of the open issues and drastically enhance application development on such networks. This survey shows the current state of the research in this domain by presenting and discussing the most representative WSN middleware. The authors concentrate on discovering similarities and differences among these approaches by making state-of-the-art classifications, and providing a framework for evaluation and appropriateness.