Scrabble is a favorite game of mine. Now Zynga has come up with their version, called Words With Friends. It's basically a Scrabble game that you can play using your iPad, iPhone, iPod Touch, Android smartphone or Android tablet. You can play against the computer, but it's more fun when you play against friends. You can play while sharing one device (I've been playing on an Android tablet, pretty fun!), or you can play against others online. This is the cool thing about the game, is that two people can play against each other anywhere, and it is cross-platform. I've been playing online using an Android phone against friends with iPhones and iPads, as well as other Android devices. We have a couple of ongoing games here at the office. Making it even more social, you can play against random players. The system matches you up with another player at random. You can even send text messages back and forth while playing, even with anonymous players.
I currently have about 10 games going simultaneously. It's fun, because there's no time limit. Some games sit idle for days, others you may find have people on them constantly. I've rematched with several of the random players because the games are challenging and really fun. Since it is semi-social, it's pretty addictive. It will definitely test your word skills and keep you on your toes!
The way they keep it free is with ads, which usually show up after every word you place. It has a solid vocabulary, with a lot of words you don't even expect are words (helpful when you're running out of letters). Keep a dictionary handy (or an online one) and you'll be working on some new words for yourself, as well.
Look for Words With Friends on the Apple App store or on the Android Market. Perhaps you'll be my next opponent!