NuGet is a Visual Studio extension that makes it easy to install and update open source libraries and tools in Visual Studio.
NuGet is a free open source developer focused package management system for the .NET platform intent on simplifying the process of incorporating third party libraries into a .NET application during development. NuGet is a member of the ASP.NET Gallery in the Outercurve Foundation.
Get started on nuget.org and find documentation on docs.nuget.org
To see NuGet in action, watch this video from MvcConf 2 and this one from DevDays 2011 Netherlands.