BusyCal is a great iCal replacement for your Mac. With the release of the 1.5.1 version, BusyCal adds support for Mailplane. This is great news, as iCal can only use Mail.app to send email.
BusyCal has much more to offer, of course. You can easily sync different calendars, there are recurring todos, live weather feeds, sticky notes, and more. But most importantly, Google Calendar integration is perfect.