Mozilla Sunbird

is a cross-platform calendar application, built upon Mozilla Toolkit. Sunbird is a standalone calendar application. It doesn’t need another client application to run (small memory requirements). Because of its standalone nature, Sunbird does not offer any kind of tight integration with an e-mail application, such as Mozilla Thunderbird.

If you need integration with Thunderbird you can use Lightning (the calendar extension for Mozilla Thunderbird). It offers calendaring features directly in the Thunderbird User Interface. Further integration features, such as e-mail invites or address book integration are planned for future releases.