Suave
Suave is a simple web development F# library
...In fact, Suave is written in a completely non-blocking fashion throughout. We have a NuGet ready for your testing needs; Suave is an excellent server for running in-process integration tests, as it's very fast to spawn. On an ordinary laptop, running hundreds of randomized tests and micro-benchmarks as well as all Suave unit tests, take about 5 seconds on mono.