The official Scrabble app by Electronic Arts will set you back $2.99 for the iPhone version. However, if you use an Android phone, the game is available for free.
It offers several ways to play including against a random opponent somewhere on the Internet, friends on Facebook, via email or pass-and-play mode with someone actually with you in person. You can even chat during game play.
Free is a relative word. Most ad-supported Android apps provide small but noticeable advertisement banners on the screen during gameplay. Scrabble does this too. However, EA took it one step too far and displays full-screen ads for other EA paid games after nearly every turn. As you might imagine, this is more than a little distracting. There does not seem to be an in-app purchase or Android Market upgrade to an ad-free version of the game. Despite these full-screen ads, Scrabble Free for Android has a respectable 3.7 average user review rating based on 1,559 reviews.