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As a result of advances in processor and network speeds, more and more applications can productively be spread across geographically distributed machines. In this paper we present a transparent system for memory sharing, InterWeave, developed with such applications in mind. InterWeave can accommodate hardware coherence and consistency within multiprocessors (level-1 sharing), software distributed shared memory (S-DSM) within tightly coupled clusters (level-2 sharing), and version-based coherence and consistency across the Internet (level-3 sharing). InterWeave allows processes written in multiple languages, running on heterogeneous machines, to share arbitrary typed data structures as if they resided in local memory. Application-specific knowledge of minimal coherence requirements is used to minimize communication. Consistency information is maintained in a manner that allows scaling to large amounts of shared data. In C, operations on shared data, including pointers, take precisely the same form as operations on non-shared data. We demonstrate the ease of use and efficiency of the system through an evaluation of several applications. In particular, we demonstrate that InterWeave's support for sharing at higher (more distributed) levels does not reduce the performance of sharing at lower (more tightly coupled) levels.