If you are looking for Thunderbird Calendar, a free alternative to Outlook Calendar, then this post may be helpful.
Thunderbird Calendar, Lightning Calendar
Mozilla Thunderbird, which is actually a Free Alternative to Microsoft Outlook for Windows, Linux etc., also supports Calendar known as Lightning Calendar, a Thunderbird extension. Through Thunderbird Calendar or Lightning Calendar, you will be able to manage your daily schedule tasks, events, meetings etc.
Share Your Meetings Through Thunderbird Lightning Calendar
Even you can share your meetings with your friends or colleagues by sending them invitations, whether they are Thunderbird Lightning Calendar users, or Outlook Calendar users. It also supports invitations from Outlook Calendar meetings or events, which will be automatically embedded with your Thunderbird Lightning Calendar after accepting those invitations.
Download Thunderbird Lightning Calendar
Check out the following website to download Lightning Calendar or search for it on Mozilla :
Lightning Calendar
You can also sync your Thunderbird Calendar with Google Calendar through Provider for Google Calendar addon. For details on Thunderbird Addons, check out the following url:
http://www.webtechquery.com/index.php/2010/03/mozilla-thunderbird-free-alternative-to-outlook/