Network emulation allows researchers to test distributed applications on diverse topologies with fine control over key properties such as delays, bandwidth, congestion, or packet loss. Current approaches to network emulation require using dedicated machines and low-level operating system support. They are generally limited to one user deploying a single topology on a given set of nodes, and they require complex management. These constraints restrict the scope and impair the uptake of network emulation by designers of distributed applications. We propose a set of novel techniques for network emulation that operate only in user-space without specific operating system support. Multiple users can simultaneously deploy several topologies on shared physical nodes with minimal setup complexity. A modular network model allows emulating complex topologies, including congestion at inner routers and links, without any centralized orchestration nor dedicated machine. We implement our user-space network emulation mechanisms in SplayNet, as an extension of an open-source distributed testbed. Our evaluation with a representative set of applications and topologies shows that SplayNet provides accuracy comparable to that of low-level systems based on dedicated machines, while offering better scalability and ease of use.