Microsoft recently introduced .NET API Browser, a one-stop shop to search all .NET APIs from a web browser. It offers an IntelliSense-like experience to find all .NET APIs without installing any developers tools. Developers can search for a namespace, class, method, or interface by typing its full or partial name directly in the API Browser page. Check out it yourself here.
Microsoft also announced that they are planning to have all .NET-compatible SDKs unified and searchable in one place. Developers can find reference documentation for .NET Framework, .NET Core, .NET Standard and Xamarin, as well as documentation for Azure NuGet packages. Check it out here.