Twitter is a great way of staying in touch with your friends, and there are some great applications that'll make this easier for you.
Some of these applications you can install and run on your desktop or laptop PC, and some you can have on you mobile phone, so here are a list of 10 examples:
twitterfeed Web Based
iPhone and iPod Touch
Other Mobile Devices with WAP and SMS
More information can also be found on the Twitter Help section.