I can't remember if I've posted this before, but anyway, check out Twitter
http://www.twitter.comIt's designed to allow you to keep in touch with what your friends are up to via sms, your browser or IM.
Say, for example, that all your friends are signed up and 'following' you on twitter, you can send one sms from your phone, IM (such as gtalk or iChat) or browser saying 'I'm going to be at Cadiz at 8pm' and all your friends will get the message (either on their cellphones or in the IM). You don't need to update everyone individually.
Sending messages from your browser or IM is free and a standard international SMS charge applies to any messages you send from your cellphone.
You can also 'follow' anyone who's twitters are set to public. This is quite cool as there are lots of weird people out there. You'll also find news services such as the BBC who send out twitters every time there is a new story. For the geeks, there's also a lot of tech people twittering from gaming conferences and tech fairs so you get to hear about the latest shit as it happens.
Anyway,,, go check it out.