The problem with trying to find any free Scrabble site is that Hasbro's lawyers are quick to take action against any website which seems to be infringing their patents and trademarks. (i.e. the makers of Scrabble don't like you playing their game, or unauthorised variations of it, unless you pay for it).
However you might like to try this site (which offers a 'practice mode'):
http://www.lexulous.com/