To build a truly responsive Unity game, you must keep long-running operations off of the main thread, and be careful to avoid blocking the main thread. This means you will need to execute various operations in the background.

Unity's coroutines are ALWAYS executed on the main Thread

Executing threads in the background implies carefully go back to the main thread when you have to manipulate Unity's objects.

Piece of cake with this package, just use this sentence:
Task.RunInMainThread(SomeFunction);
SomeFunction is guaranteed to be executed in the main thread

Of course i's possible to use lambda expressions:
Task.RunInMainThread(()=>
{
// This code will execute in the main thread
});

But this asset also makes working with threads a pleasant experience:

To create a thread just use the following syntax:
Task.Run (DoSomeWorkOnABackgroundThread);

Creating a thread that returns some value (an integer in this example):
Task<int>.Run (CalculateSomeIntegerInTheBackground),

Of course you will want to receive the integer:
Task<int>.Run (CalculateSomeIntegerInTheBackground).ContinueWith(ThisFunctionReceivesTheIntegerAsParameter);

Or, maybe, you need to show that integer to the player (the function receiving it must be executed in the main thread):
Task<int>.Run (CalculateSomeIntegerInTheBackground).ContinueInMainThreadWith(ThisFunctionReceivesTheIntegerAsParameter);

These are just the basic features included in this package.

This package reimplements and extends the .NET System.Threading.Tasks namespace.

This namespace makes thread management easier for you by adding a class named Task. A task represents an asynchronous operation. Typically, a Task is returned from an asynchronous function and gives the ability to continue processing the result of the task.
A task is not tied to a particular threading model: it represents the work being done, not where it is executing. Tasks have many advantages over other methods of asynchronous programming such as callbacks and the event model.

You can learn more about tasks in Parse's website

Tested for standalone builds, web player, iOS and Android

Full source code is included, as well as many examples. I use this asset for my own projects, so I will keep improving and expanding it.

If you have any comments, doubts, feature requests or just need some help implementing some multithreading behaviour just let me know in the forum's thread