The .NET ecosystem keeps evolving, and 2026 is no exception. Whether you're building APIs, CLIs, or cloud-native microservices, the right open source library can save you weeks of work. Here are 10 libraries that are dominating NuGet downloads and GitHub stars this year—spanning battle-tested essentials to rising stars you'll want on your radar. 1. Polly — Resilience Made Fluent If your app talks to external services, you need Polly. It provides retry, circuit breaker, timeout, bulkhead isolation, and fallback policies—all composed in a fluent API. var pipeline = new ResiliencePipelineBuilder < HttpResponseMessage >() . AddRetry ( new RetryStrategyOptions < HttpResponseMessage > { MaxRetryAttempts = 3 , Delay = TimeSpan . FromMilliseconds ( 500 ), BackoffType = DelayBackoffType . Exponential }) . AddCircuitBreaker ( new CircuitBreakerStrategyOptions < HttpResponseMessage > { FailureRatioThreshold = 0.5 , SamplingDuration = TimeSpan . FromSeconds ( 30 ) }) .…