There are a lot of weird texting applications for the iPhone, and some of them are pretty hinky in terms of design or function. But Texto is probably the best looking SMS app I've seen -- it doesn't do any weird non-SMS text messaging or have all kinds of neon graphics running around. All it does is hook up with your iPhone's standard texting service to let you get quick shortcuts to your most commonly used people and messages. Thanks to LA traffic, I'm often running just a little late for meetings, and with Texto, I can just tap a few buttons to send an "I'll be there soon" to my friends and colleagues rather than trying to type the whole thing out as I frantically drive across town.
The app's only 99 cents, but I'll admit it's not for everybody. If you don't text a whole lot already, you probably don't need a whole other app to handle your most frequently-sent messages. And obviously, sending a pre-written message is a little impersonal. Your significant other might realize you're blowing them off the fifth time you send a "Sorry, I'm busy, call you soon," message to them. There's one other caveat: you can only use this one in iOS 4.0, due to the way it interacts with the SMS app.
For the right user, Texto can be really helpful, and it's got the bonus of having a great and simple interface that's free of ads and clutter.