I just tried twitter last week and was blown away! This is one really really cool service.
For me the coolest part about it is the SMS/Texting feature. For those who haven't used it yet, think of it as a service which lets you SMS/Text all your friends by sending an SMS to just a single number that twitter gives.
Twitter then goes out and updates everyone on your list by sending them SMS plus updating a whole lot of other places like your Facebook profile, your blog...
Other features include sending the message from the Twitter website or even from a downloadable app (imagine that FREE SMS), if you want to you can send a message to just one friend.
Note to Sri Lankan users, this works find on Dialog but a few of my friends on Mobitel had trouble getting setup. If any of you get this working with Mobitel please let me know.
Once you have created a twitter account go to http://twitter.com/merill and 'follow me' to get my twitter updates.