I've just downloaded and tried both What The Doodle and Sketch Online Beta and played them both for a while.
While Sketch Online is Free, has a better interface, better drawing tools and more players. Most all of the players cheat by drawing the word your supposed to guess at the last few seconds and reply "If You Don't Like It, Leave." if you complain. I think it's because the scoring system rewards the drawer for the number of people who guess correctly, so it begs to be exploited. Also, the chat is dominated by myspace types who post photos of their abs and cleavage as avitars and try to talk-up each other.
Though What The Doodle isn't free and isn't made as well, it still does what it's supposed to do and you actually have a bigger space to draw. The application price (about $1.50) seems to keep the cheaters and myspacers out, which despite the flaws made the game more fun to me. Cheating just ruins it. In this game the round is over when the 1st person guesses correctly, so it doesn't encourage cheating as much. I recommend What The Doodle.