Overlay networks have emerged as a powerful and highly flexible method for delivering content. We study how to optimize throughput of large transfers across richly connected, adaptive overlay networks, focusing on the potential of collaborative transfers between peers to supplement ongoing downloads. First, we make the case for an erasure-resilient encoding of the content. Using the digital fountain encoding approach, end-hosts can effectviely reconstruct the original content of size n from a subset of any n symbols drawn from a large universe of encoded symbols. Such an approach affors reliability and a substantial degree of application-level flexibility, as it seamlessly accomodates connection migration and parallel transfers while providing resilience to packet loss. However, since the sets of encoded symbols acquired by peers during downloads may overlap substantially, care must be taken to enable them to collaborate effectively. Our main contribution is a collection of useful algorithmic tools for efficient estimation, summarization, and approximate reconciliation of sets of symbols between collaborating peers, all of which keep message complexity and computation to a minimum. Through simulations and experiments on a prototype implementation, we demonstrate the performance benefits of our informed content delivery mechanisms and how the complement existing network overlay architectures.