Zynga may well be one of the fastest growing companies that you've never heard of. And if you haven't heard of Zynga, you've probably interacted with one of its Facebook, iPhone or web apps. Lots of other developers and companies compete in their space... and many do very well. But Zynga has really mastered the UI. Today, they launched Mafia Wars for the iPhone (already a popular application on Facebook). In and of itself, this isn't news. There are remarkably similar games available... I even remember playing the same game on my TI-83 calculator in high school.
What is noteworthy is how terrific Zynga's artwork, user experience and development are. The game is gorgeous. The UI is exceptional. And this is how Zynga makes a not-so-unique game stand out.
It is also how Zynga makes other games and applications stand out: they get viral behavior and they understand how it translates into gaming applications. Scramble and Poker are not huge successes because the games are so differentiated - they are successful because Zynga taps into your social graph and masterfully layers in incentives.
By building terrific games that are weaved into your social graph, Zynga attracts the users. That 'social stickiness' allows Zynga to profit from the users' willingness to upgrade, advance their characters, and differentiate from the masses.