There are also Continuous LINQ [1][2], Bindable LINQ [3], Obtics [4] and OLinq [5], some predating Rx [6][7]. Besides OLinq they all seem no longer maintained but in case somebody is interested in other existing implementations, there you go. Also note that they don't all solve the exact same but similar problems.

[1] http://clinq.codeplex.com [2] https://github.com/ismell/Continuous-LINQ [3] https://bindablelinq.codeplex.com [4] http://obtics.codeplex.com [5] https://github.com/wasabii/OLinq [6] https://msdn.microsoft.com/en-us/data/gg577609.aspx [7] https://github.com/Reactive-Extensions/Rx.NET

This is not a replacement for Rx, actually it relies on Rx. SmartReactives is an addition to Rx which allows you to get an IObservable from an expression for free, while with just Rx you'd have to instrument your expression.

There are many application-specific scenarios that are enabled by RX that would still take quite a bit of hand-rolling a deep knowledge of RX to get right.

A favorite of mine right now is https://github.com/RolandPheasant/DynamicData which I use for easily "deriving" from in-memory collections to create new collections that are actually "live views" onto the source collection. Makes things like mapping, sorting, filtering, paging, and so on super-simple while letting you avoid creating unnecessary state.

Rx.net is probably the most well used one.

I like componentized functionalities a lot, and i think .NET shines for that.