Lexulous. There used to be a Scrabble game on Facebook, until the owners of Scrabble got sniffy about it. So the authors gave it a bigger board (and tileset). There is also now an 'approved' Scrabble, from somebody else, but a lot of us fell in love with Lexulous. You don't have to play via FB, as Lexulous has its own website, but I think most of the people I play do go via FB.