RadEditor for ASP.NET AJAX
Contents
RadControls for ASP.NET AJAX

Here is an example demonstrating how to create a line spacing dropdown, from which you can change line distance within text:

CopyASPX
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<telerik:RadEditor runat="server" ID="RadEditor1" OnClientCommandExecuting="OnClientCommandExecuting">
    <Tools>
        <telerik:EditorToolGroup>
            <telerik:EditorDropDown Name="LineSpacing" Text="Line Spacing" PopupWidth="110" PopupHeight="90">
                <telerik:EditorDropDownItem Name="Default" Value="" />
                <telerik:EditorDropDownItem Name="Single space" Value="20px" />
                <telerik:EditorDropDownItem Name="1.5 Lines" Value="35px" />
                <telerik:EditorDropDownItem Name="Double spacing" Value="50px" />
            </telerik:EditorDropDown>
        </telerik:EditorToolGroup>
    </Tools>
</telerik:RadEditor>
<script type="text/javascript">
    function OnClientCommandExecuting(editor, args)
    {
        var name = args.get_name();
        var val = args.get_value();
        var cArea = editor.get_contentArea();
        if (name == "LineSpacing")
        {
            if (cArea.firstChild && cArea.firstChild.tagName == "P")
            {
                cArea.firstChild.style.lineHeight = val;
            }
            else
            {
                editor.set_html("<p style='line-height:" + val + "'>" + editor.get_html() + "</p>");
            } 
            args.set_cancel(true);
        }
    }
</script>