How to output a variable value to the log from MSBuild

You can actually debug MSBuild scripts with Visual Studio 2010 now. It requires some hacking, and it isn't officially supported, but it is an option.

Otherwise use the Message task. Normal rules for referencing Properties, Items and Item Metadata (also referred to as batching) apply.

This example:

<Project DefaultTargets="Build"
         xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup>
    <TestItem Include="test1" />
    <TestItem Include="test2" />
    <TestItem Include="test3" />
  </ItemGroup>

  <PropertyGroup>
    <TestProperty>Property Value</TestProperty>
  </PropertyGroup>

  <Target Name="TestMessage" AfterTargets="Build" >

    <!-- Use $(Property Name) to reference a property -->
    <Message Text="$(TestProperty)" Importance="high"/>

    <!-- Use @(Item Name) to output a semi-colon
         separated list of items on one line      -->
    <Message Text="@(TestItem)" Importance="high"/>

    <!-- Use %(Item Name.Metadata Property Name) to 
         call the Message task once for each item.   -->
    <!-- This will output each item on a separate line -->
    <Message Text="%(TestItem.Identity)" Importance="high"/>

  </Target>
</Project>

Will produce this output:

Property Value
test1;test2;test3
test1
test2
test3