Many peer-to-peer (P2P) systems require accurate information about their peer's available bandwidth, e.g., for load balancing. Determining this information is difficult, as a suitable approach must address two challenges. First, it must be able to deal with fluctuating bandwidth. Second, it must incur low overhead to prevent interference with the operation of the P2P system. In this paper we present an approach to estimate the available bandwidth of peers in a P2P system, based on a combination of traffic observation and the strategic injection of traffic into the system. We evaluate our approach and show that it is accurate and responsive in settings with variable bandwidth while resulting in limited interference with the system.