InstaCal is an affordable, yet powerful calendar app that puts all your events right at your fingertips, always available in your Mac's menu bar. InstaCal is quick and convenient, and can be opened at any time with a configurable keyboard shortcut. From there you can view your calendar events, make changes, invite friends or even add new events.
You can use any of the calendars added to your Mac (those available in the Calendar app), or connect directly with Google Calendar (even with multiple accounts!) to utilize additional features like invitations and viewing responses to your invitations.