Reactive Extensions (Rx) is now open source and supports LINQ styles in C++.

From the announcement, Rx is already being used for a number of highly responsive applications including GitHub for Windows:

According to Paul Betts at GitHub, "GitHub for Windows uses the Reactive Extensions for almost everything it does, including network requests, UI events, managing child processes (git.exe). Using Rx and ReactiveUI, we've written a fast, nearly 100% asynchronous, responsive application, while still having 100% deterministic, reliable unit tests. The desktop developers at GitHub loved Rx so much, that the Mac team created their own version of Rx and ReactiveUI, called ReactiveCocoa, and are now using it on the Mac to obtain similar benefits."

In addition to Rx's original support for .NET and new support for Javascript, Rx has also added support for C++:

Rx++: The Reactive Extensions for Native (RxC) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators in both C and C++.

Ix++: An implementation of LINQ for Native Developers in C++.

Where to download or find out more: https://rx.codeplex.com/