In my (relatively) new job I am booking international web conferences and phone calls on an almost daily basis. One of the easiest tools on the internet is the World Clock Meeting Planner: http://www.timeanddate.com/worldclock/meeting.html
So easy to use, simply put in the cities or time zones that you need and hey presto, it will find the best time to arrange that phone call. As an example, I’ve taken a screen shot of an example, between Sydney and Berlin. The red sections are when most people are asleep, and the green sections are the best times (i.e. during the day).
Do you use other tools for this type of thing? Feel free to share suggestions in the comments.