There is a lot to be said about doing one thing very well. Services like Twitter started as a way to update friends on what you are doing and it has stayed true to its mission. Other services have started simple and have fallen victim to feature creep and trying to be everything to everyone.
I am particularly fond of online applications that have some focus, use the medium very well and extend the focus in strategic, well-planned ways.
Take this new site called Umbrella Today. If you go to the site and enter your zip code, it tells you whether or not you need an umbrella that day. Super simple, very useful and they extend it to mobile very logically and at the right point in the interaction.
Once you see if you need an umbrella, they offer you an option to see if you would like an SMS alert should you need an umbrella in the future.
Simplification of complex systems and applications is a niche market in itself. This is a perfect example, think about how many clicks and how much reading it takes you to find out the answer to this simple question on a weather site.
There are very few sites that can maintain their focus, but those that do remain useful and relevant. What examples of simple websites or programs do you love?