Twitter offers something I always expected from ICQ years ago - back in the days when ICQ and Yahoo were the only ones to offer IM. I remember a time when ICQ setup a system where you could send sms messages to your mobile, and back to ICQ, enabling you to ICQ through sms and staying in touch in that way. It could be very handy at times, when you had to go out but wanted to continue a conversation. Unfortunately, most mobile providers blocked this due to earnings they did not make on this service.
Twitter does this, but web 2.0 style : You register simply by entering your mobile number and your name, and you're ready to go. From that moment on, you can enter on the site what you're up to or what you'd like to share, or you can send sms messages to a number, and your message will immediately appear in the message queue. It's possible to set privacy to the messages so they get filtered out of the public loop, and there's RSS on your message 'stream', as well as a java applet you can place on your website. All in all : Very useful and it works without having to install anything, configure anything or whatsoever.
Twitter with me !