Latest Mozilla Sunbird is a well-connected calendar
Mozilla's Sunbird calendaring application lives perpetually in the shadow of its siblings Firefox and Thunderbird, garnering just a fraction of the developer effort and publicity lavished on the browser and email client. Nevertheless, it is slowing maturing into a reliable tool worthy of the Mozilla brand.
You can test-drive Sunbird in two ways: either with the standalone Sunbird application (available in official binaries for Windows, x86 Linux, and Mac OS X), or with the Lightning extension for Thunderbird (which comes as an XPI cross-platform install). The two applications are developed concurrently; the current releases of both are numbered 0.5. Lightning 0.5 will install into Thunderbird version 1.5 or later. To install Lightning, download the installer using Save as ... in your Web browser, then open it from within Thunderbird's Tools -> Add-ons menu.