msbuild.xml

<Project DefaultTargets="Default" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <Foo Condition="$(Foo)==''">bar</Foo>
  </PropertyGroup>

  <Target Name="Default">
    <Message Text="$(Foo)" />
  </Target>

</Project>

<!--
 invoke with
 msbuild msbuild.xml /p:Foo=baz
-->

rakefile.rb

foo=ENV['foo'] || 'bar'

task :default do
   puts "foo is #{foo}"
end

#invoke with 
# rake foo=baz