Most of what's attracted me to the site has been their very simple API, which has allowed me to get an application called "Twitterer" up and running in less than a few hours (1, 2, 3). I've not yet released it (no icon!) but i've been using it for the past two days and am quite pleased with it (if I may say so myself). In fact, if you have read this before, you might notice a recent trend in posts about webservices...ahem.
Twitter allows the masses to finally answer the question "what are you doing?"
To which the masses can reply in a most resounding fashion "nothin really."