iCal is a great way to keep track of your schedule and appointments, and is included in every copy of Mac OS X. A lot of people use iCal in their everyday life to make a note of important events or set alarms and reminders. It also syncs with all your iOS devices so you can keep track of these events on the go.
Replacements for iCal will never match up to it because iCal has now become an industry standard way for sharing schedules, contacts and such.
Calender is a freeware app for Mac OS X that allows you to access iCal from your menu bar. Though iCal is not a heavy application, launching it everytime you want to see the appointments for a particular date or update something can get irritating.
Calender has a nice minimalistic presence in your menu bar, and to access the calender for the current month, you can just click on the icon. Based on iCal’s multiple calender support, Calender also supports the color coding for various calenders (Home, Work etc). These color coded events are displayed when you hover over a particular date that has events.
When you click on a date, Calender quickly brings up the iCal page for that day so you can edit or rearrange tasks for that day.
Overall, for those who want quick access to their calender at all times, Calender.app is a great tool.