Add a calendar to your e-mail client.
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Free
Lightning is an extension for Mozilla Thunderbird that adds an integrated calendar to the e-mail client.
It is based on the standalone Mozilla Sunbird calendar application. Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
DOWNLOAD
Free