Planning a meeting or phonecall across multiple timezones can be tough.
I've used a few online tools, but my current favourite is a gnome app called slashtime by Andrew Cowie. I saw Andrew speak at Linux Conf Au in 2009 on GUI design, using slashtime as an example, and it was a great talk.
Highly recommend. Would be great to see a debian package so it is apt-gettable.