TinyCal is a calendar application with Google Calendar support. There are two reasons why you would want to replace iCal with TinyCal. The first one of them is that TinyCal runs from your MenuBar and it gives you really quick access to your calendar. The second one is that TinyCal can be synced with Google Calendar, and from what I hear that feature works well.
TinyCal runs from the system menu, and it shows a badge with the current day of the month. If you click on the icon, a small square window will appear which shows the current calendar for the month. You can use the arrow buttons to go back and forth to previous and next months. Days that have events on them will be highlighted in a darker color.
The version of TinyCal that I tried seemed to have a few problems and it has been recently pulled from the App Store. In my case, the main problem was that I couldn't use the feature to add new events. The button was not available and I read some user reviews on the App Store which complained about that same bug.
I would like TinyCal to automatically pull my calendar data from iCal if that were possible, too.