To avoid link rot, here are the feed urls for the official feeds:

**For all versions up to and including Sitecore 9.0:**

NuGet V3 feed URL (Visual Studio 2015+)

- https://sitecore.myget.org/F/sc-packages/api/v3/index.json

NuGet V2 feed URL (Visual Studio 2012+)

- https://sitecore.myget.org/F/sc-packages/

Browse packages

- https://sitecore.myget.org/gallery/sc-packages

**For Sitecore 9.1, there is a version specific feed:**

NuGet V3 feed URL (Visual Studio 2015+)

 - https://sitecore.myget.org/F/sc-platform-9-1/api/v3/index.json

NuGet V2 feed URL (Visual Studio 2012+)

 - https://sitecore.myget.org/F/sc-platform-9-1/api/v2

Browse packages

- https://sitecore.myget.org/gallery/sc-platform-9-1


If you are using a build server, as well as adding this to Visual Studio you will want to update your `Nuget.config` file:

<!-- language-all: xml -->

    <?xml version="1.0" encoding="utf-8"?>
    <configuration>
      <!--
      Used to specify the default Sources for list, install and update.
      -->
      <packageSources>
        <add key="Official Sitecore" value="https://sitecore.myget.org/F/sc-packages/api/v3/index.json" />
      </packageSources>

      <activePackageSource>
        <!-- this tells that all of them are active -->
        <add key="All" value="(Aggregate source)" />
      </activePackageSource>
    </configuration>


There is a nice post here talking about adding that to your VS solution: https://jermdavis.wordpress.com/2016/09/05/the-official-sitecore-nuget-feed-is-here/

Some of the main points when using are:
* There are packages with `.NoReferences` suffixed. These are probably the most useful as they don't try to bring in a lot of dependencies you may not need in your project.
* They set the binaries as `CopyLocal=true`, so you may want to change that in your project

@kamsar wrote a nifty PowerShell script to update your solution to use this: http://kamsar.net/index.php/2016/09/Nugetify-your-Sitecore-References/