After a friend told me about how useful google calendar is (I had heard of it before, just never looked at it), I finally went and registered at google to have a look myself. For a web application the gui is pretty good. What I liked most about it, is that I can easily synchronize the entries in google calendar with my calendars in thunderbird or my cell phone. For my cell phone I use MyPhoneExplorer (already had that installed, I use it to write SMS via a bluetooth connection to my computer). For thunderbird I use to addons. The first is a calendar addon called Lightning since thunderbird has no calendar out-of-the-box. The second is an addon written to sync between google calendar and thunderbird called Provider.
Everything was pretty straight-forward to setup, took me less than half an hour to find information, get everything installed and running smoothly.
Links:
Google Calendar http://www.google.com/calendar/
Provider https://addons.mozilla.org/de/thunderbird/addon/4631
Lightning https://addons.mozilla.org/de/thunderbird/addon/2313
MyPhoneExplorer http://www.fjsoft.at/de/downloads.php