Synchronize calendar between Mac, Android mobile and Windows 7
Your best option is probably Google Calendar. It is an "online solution", but you don't have to make use of the webapp at all, just use it as a method to sync.

For OSX, you can use iCal synced to Google Calendar using CalDAV. It's not perfectly polished, but it works fine. You may need to enable any other calendars via the Delegation tab, under Accounts in the iCal preferences.

I'm not an Android guy, but I think Google Calendar support should be automatically set up when you activate your device with a Google account.

For Windows 7, it depends what app you're wanting to use. It looks like they removed the Calendar app from Vista. The simplest option may be this desktop gadget that syncs to Google Calendar. If you're looking to use other software, Google provides a sync option for Outlook, or you can use anything that supports the CalDAV standard.