Although NuGet ships with ASP.NET MVC 3 and complements it nicely, it's not restricted to ASP.NET MVC by any means. NuGet can be used to install packages for nearly any type of project within Visual Studio. Building a Windows Phone application? There's a set of NuGet packages for it.
But when you are building an ASP.NET MVC 3 application, NuGet is a great companion. Many packages are available that take advantage of specific features built into ASP.NET MVC.
For example, you can install the Ninject.Mvc3 package, which automatically wires up the Ninject dependency injection library as the dependency resolver. Installing the MvcScaffolding package adds new scaffold templates to the Add Controller dialog.
And when you are ready to share your own useful libraries with the world, don't just place them in a zip file and pop them on the Web. Turn them into a NuGet package and make it easy for others to discover the great work you've created.