Wow. Ofcourse Google Calendar has this. What am I talking about? Google Calendar now let you use an iframe to show your calendar on your own website so that people can see when you are busy or have free time.
Want to learn more about embedded Google Calendar to put on your own website? Then look here: http://www.google.com/calendar/embed/EmbedHelper.html
I made a test for myself here: http://kservik.com/kalender-kse-neteffects.html