There’s 3 approaches :).
In NuGet 1.1 (The latest release) we’ve improved powershell pipelining so you can do this:
Get-Project -All | Install-Package SomePackage
That will install “SomePackage” into all of your projects. You can use wildcards to narrow down which projects:
Get-Project Mvc* | Install-Package SomePackage
That will use wildcard semantics (in this case, find all projects that start with mvc).
Get-Project SomeProject | Install-Package SomePackage
That will install SomePackage into SomeProject and nothing else.