orleans

Orleans is Microsoft's framework for building distributed systems in C# without doing the distributed-systems homework. Write ordinary classes with async methods (grains), and the runtime spreads them across a cluster, handles placement, recovers from failures, and scales. It invented the virtual actor model: grains always logically exist, so a server crash never invalidates a reference. This is the tech behind Halo and Xbox services. MIT licensed, free.

Running it means hosting your own silo cluster: pick a clustering provider (SQL, Redis, Azure Tables, Consul, Cassandra) and storage providers for grain state, then deploy to Kubernetes, Azure, or your own VMs. Version 10 added a built-in dashboard for watching the cluster live, and distributed ACID transactions across grains with no central coordinator remain a feature almost nothing else in the category offers.

Use it if you're a .NET shop building stateful, real-time, or high-concurrency backends: game services, IoT, financial systems. Akka.NET is the classic-actor alternative, Dapr is the answer for polyglot teams, and Temporal covers the durable-workflow overlap.

The catch: .NET-only, a real learning curve on the actor model, and major versions have broken things before (v7 replaced the entire serialization system). Its popularity metrics badly understate adoption. Judge it by the production list, not the popularity contest.