When Facebook gaming giant Zynga bought Newtoy and rebranded it as Zynga with Friends, it was only a matter of time until games from the "with Friends" franchise started popping up on Facebook. Sure enough, Words with Friends is now playable on Facebook as of a couple weeks ago. The game's interface is basically identical to the iOS games, so if you're already familiar with those, you won't need to adapt much to the Facebook game.
The coolest feature by far is the game's integration with the mobile versions. Playing the browser-based version will sync game progress to Words with Friends on