SMS messages have a 160-character limit, but the iPhone doesn't have an on-screen character counter to let you know when you're nearing the limit. txtctr is a simple web application to stop you running off at the keyboard.
Application author Tom Dance explained the genesis of the application in an email to Lifehacker:
The iPhone has no char counter when composing an SMS, so I created a light and simple web app. The idea is that it can just sit open in mobile Safari all the time, then when you are writing a message you can copy and paste it into the web app and it will tell you how many chars you have. This is very useful if you're on a pre-paid plan and don't want to be sending multiple messages when you're only a few chars over the 160 limit.
Lifehacker's Australian editor isn't an iPhone user, so I haven't tested the app in situ; if you try it out, let us know how it works in the comments. One limitation Tom points out: if you paste text into the web app, you need to type another character to update the count.