A nice Ant target to print out all the Ant's built-in properties:

<target name="echo" >
    <echo message="os.name:          ${os.name}"          />
    <echo message="basdir:           ${basedir}"          />
    <!-- absolute path of the build file. -->
    <echo message="ant.file:         ${ant.file}"         />
    <!-- path to executing ant's root directory. -->
    <echo message="ant.home:         ${ant.home}"         />
    <echo message="ant.version:         ${ant.version}"   />
    <echo message="ant.project.name: ${ant.project.name}" />
    <echo message="ant.java.version: ${ant.java.version}" />
    <!-- System properties. -->
    <echo message="user.home:         ${user.home}"   />
    <echo message="user.name:         ${user.name}"   />
    <echo message="java.home:         ${java.home}"   />
</target>

Originally found here.