Google Calendar lets you add calendars listing useful dates, and it has offered a list of Australian public holidays for quite a while. That service is being expanded this week, with calendars added for 30 more international destinations.
To add public holiday calendars from the web interface, click on the "Other calendars" dro-down in the left-hand column, select "Browse Interesting Calendars" and choose the "Holidays" tab. You can preview calendars or subscribe to them from here.
If you're planning an overseas trip, it's definitely worth checking out if there are any local public holidays -- it can make a big difference to what's open, public transport timetables and general levels of crowding.
New holiday calendars [Official Gmail Blog]