Selection.parent

This will permit to append childs (siblings between them) to a element in chaining (without set a var with the parent).

Now:
var div = d3.select( 'body' ).append( 'div' );
div.append( 'p' ).text( 'I\'m a P' );
div.append( 'a' ).text( 'I\'m an A' )
        .append( 'span' ).text( 'with a SPAN' );
div.append( 'span' ).text( 'I\'m just a SPAN' );

With this:
d3.select( 'body' )
    .append( 'div' )
        .append( 'p' ).text( 'I\'m a P' )
    .parent( )
        .append( 'a' ).text( 'I\'m an A' )
            .append( 'span' ).text( 'with a SPAN' )
        .parent( )
    .parent( )
        .append( 'span' ).text( 'I\'m just a SPAN' );