Appointy looks like the best featured appointment plugin available here on the WP.org repository. However, it appears it hasn't been updated in some time. It's showing compatibility to version 3.0.5 of WordPress and version 3.4 could come out at any time now. I'm not excited to install it and test it because of some of the changes that have happened in WP since 3.0.
Is it still being developed? Is it compatible with 3.3.3?