Is it possible to write custom text on Google Maps API v3?

To show custom text you need to create a custom overlay. Below is an example adapted from official Google documentation. You could also use this library for more "stylish" info windows

<html>

<head>
  <script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false">
  </script>
  <script>
    //adapded from this example http://code.google.com/apis/maps/documentation/javascript/overlays.html#CustomOverlays
     //text overlays
    function TxtOverlay(pos, txt, cls, map) {

      // Now initialize all properties.
      this.pos = pos;
      this.txt_ = txt;
      this.cls_ = cls;
      this.map_ = map;

      // We define a property to hold the image's
      // div. We'll actually create this div
      // upon receipt of the add() method so we'll
      // leave it null for now.
      this.div_ = null;

      // Explicitly call setMap() on this overlay
      this.setMap(map);
    }

    TxtOverlay.prototype = new google.maps.OverlayView();



    TxtOverlay.prototype.onAdd = function() {

      // Note: an overlay's receipt of onAdd() indicates that
      // the map's panes are now available for attaching
      // the overlay to the map via the DOM.

      // Create the DIV and set some basic attributes.
      var div = document.createElement('DIV');
      div.className = this.cls_;

      div.innerHTML = this.txt_;

      // Set the overlay's div_ property to this DIV
      this.div_ = div;
      var overlayProjection = this.getProjection();
      var position = overlayProjection.fromLatLngToDivPixel(this.pos);
      div.style.left = position.x + 'px';
      div.style.top = position.y + 'px';
      // We add an overlay to a map via one of the map's panes.

      var panes = this.getPanes();
      panes.floatPane.appendChild(div);
    }
    TxtOverlay.prototype.draw = function() {


        var overlayProjection = this.getProjection();

        // Retrieve the southwest and northeast coordinates of this overlay
        // in latlngs and convert them to pixels coordinates.
        // We'll use these coordinates to resize the DIV.
        var position = overlayProjection.fromLatLngToDivPixel(this.pos);


        var div = this.div_;
        div.style.left = position.x + 'px';
        div.style.top = position.y + 'px';



      }
      //Optional: helper methods for removing and toggling the text overlay.  
    TxtOverlay.prototype.onRemove = function() {
      this.div_.parentNode.removeChild(this.div_);
      this.div_ = null;
    }
    TxtOverlay.prototype.hide = function() {
      if (this.div_) {
        this.div_.style.visibility = "hidden";
      }
    }

    TxtOverlay.prototype.show = function() {
      if (this.div_) {
        this.div_.style.visibility = "visible";
      }
    }

    TxtOverlay.prototype.toggle = function() {
      if (this.div_) {
        if (this.div_.style.visibility == "hidden") {
          this.show();
        } else {
          this.hide();
        }
      }
    }

    TxtOverlay.prototype.toggleDOM = function() {
      if (this.getMap()) {
        this.setMap(null);
      } else {
        this.setMap(this.map_);
      }
    }




    var map;

    function init() {
      var latlng = new google.maps.LatLng(37.9069, -122.0792);
      var myOptions = {
        zoom: 4,
        center: latlng,
        mapTypeId: google.maps.MapTypeId.ROADMAP
      };
      map = new google.maps.Map(document.getElementById("map"), myOptions);

      var marker = new google.maps.Marker({
        position: latlng,
        map: map,
        title: "Hello World!"
      });

      customTxt = "<div>Blah blah sdfsddddddddddddddd ddddddddddddddddddddd<ul><li>Blah 1<li>blah 2 </ul></div>"
      txt = new TxtOverlay(latlng, customTxt, "customBox", map)

    }
  </script>
  <style>
    .customBox {
      background: yellow;
      border: 1px solid black;
      position: absolute;
    }
  </style>
</head>

<body onload="init()">
  <div id="map" style="width: 600px; height: 600px;">
  </div>
</body>

</html>

By far the easiest way to add a Text Overlay is to use the MapLabel class from https://github.com/googlemaps/js-map-label

var mapLabel = new MapLabel({
  text: 'Test',
  position: new google.maps.LatLng(50,50),
  map: map,
  fontSize: 20,
  align: 'right'
});

To add custom text to google maps, you can use a marker with an empty pixel for the icon:

new google.maps.Marker({
    position: { lat: 0, lng: 0 },
    map: map,
    icon: '../res/img/empty.png',
    label: {
        color: '#FF0000',
        fontWeight: 'bold',
        text: 'Hello World',
        fontSize: '20px',
    },
});

Style options are listed here