Gnutella is still one of the most popular P2P systems with millions of users. The advantages of Gnutella are its low maintenance overhead, its excellent robustness properties, and its query processing flexibility. Recent improvements, such as the introduction of ultrapeers and augmented node degrees also significantly reduced its excessive network bandwidth usage which was one of Gnutella's major drawbacks. Despite these improvements, Gnutella is still inefficient for rare queries in terms of low success rates and massive message propagation overhead. In this paper we augment the unstructured Gnutella network with a structured overlay network of ultrapeers based on the Kademlia DHT to address the problem of rare queries in Gnutella. We present the required query, maintenance, and ultrapeer election algorithms which use both overlays at their optimal efficiency, describe the protocols and architecture of our hybrid system, and present our implementation on the basis of the LimeWire Gnutella client and the Azureus Kademlia implementation. To demonstrate the advantages and efficiency of our hybrid approach we provide experimental results from large-scale experiments with hybrid ultrapeers running on PlanetLab which were connected to the live LimeWire Gnutella and Azureus Kademlia networks, with approximately 4 million (LimeWire) and 800 thousand (Azureus) connected users during the experiments.