Source

<html>
  <head>
    <title>Step 10: add a marker</title>
    <script src="../OpenLayers.js"></script>
    <script src="../OpenStreetMap.js"></script>
    <script src="http://maps.google.com/maps/api/js?sensor=false"></script>
    <link rel="stylesheet" href="../theme/default/style.css" type="text/css" />

    <style type="text/css">
      .olControlAttribution {
        bottom: 0px;
        right:  220px;
        background-color:white; 
        padding: 4px; 
        opacity: 0.60;
      }
      .olControlScaleLine {
        bottom: 0px;
        left: 0px;
        background-color:white; 
        padding: 4px; 
        opacity: 0.60;
      }
      .olControlPermalink {
        top: 0px;
        right: 0px;
        height: 1.2em;
        background-color:white;
        padding: 4px;
        opacity: 0.60;
    }
    </style>

    <script>
      function init() {
        lat       = 52.025;
        lon       = 8.515;
        zoom      = 15;

        projLonLat   = new OpenLayers.Projection("EPSG:4326");   // WGS 1984
        projMercator = new OpenLayers.Projection("EPSG:900913"); // Spherical Mercator

        extentMax = 20037508.34;

        overviewMap = new OpenLayers.Control.OverviewMap();

        scale = new OpenLayers.Control.ScaleLine();
        scale.geodesic = true; // get the scale projection right, at least on small zoom levels

        map = new OpenLayers.Map("demoMap", 
                                 { theme:    null,
                                   controls: [ new OpenLayers.Control.Navigation(),    // direct panning via mouse drag
                                               new OpenLayers.Control.LayerSwitcher(), // select map and features to display
                                               new OpenLayers.Control.Attribution(),   // attribution text
                                               new OpenLayers.Control.PanZoomBar(),    // larger navigation control
                                               new OpenLayers.Control.Permalink(),     // bookmarkable map links
                                               scale,                                  // scale ruler
                                               overviewMap                             // overview map
                                             ],
	                           maxExtent: new OpenLayers.Bounds(-extentMax, -extentMax, extentMax, extentMax)
                                 } 
                                );
        map.addLayer(new OpenLayers.Layer.OSM.Mapnik("Mapnik"));
        map.addLayer(new OpenLayers.Layer.OSM.Osmarender("Osmarender"));
        map.addLayer(new OpenLayers.Layer.Google("Google (Map)",
            { 'type': google.maps.MapTypeId.ROADMAP, 'sphericalMercator': true } ));
        map.addLayer(new OpenLayers.Layer.Google("Google (Hybrid)",
            { 'type': google.maps.MapTypeId.HYBRID, 'sphericalMercator': true } ));
        map.addLayer(new OpenLayers.Layer.Google("Google (Satellite)",
            { 'type': google.maps.MapTypeId.SATELLITE, 'sphericalMercator': true } ));
        map.setCenter(new OpenLayers.LonLat(lon, lat).transform(projLonLat,projMercator), zoom);
        overviewMap.maximizeControl();

        markers = new OpenLayers.Layer.Markers("Markers");
        map.addLayer(markers);
        markers.addMarker(new OpenLayers.Marker(new OpenLayers.LonLat(lon,lat).transform(projLonLat, projMercator)));
      }
    </script>
  </head>
  <body onload="init();">
    <div id="demoMap"></div>
  </body>
</html>