The Lightning add-on for the open-source e-mail client Thunderbird (for Windows and Mac) updates officially to Version 0.9, and Mozilla promises that it will be the last one before it's rolled into Thunderbird 3.0.
This update doesn't introduce quite as many new features as the last one did, but there are still plenty of major changes. Interface enhancements include a visual indicator for events that span multiple days, an overhaul of both the "minimonth" calendar in the upper left and the main day/week/multiweek/month calendar views, a progress indicator for when remote calendars are loading, and support for the Today pane in both calendar mode and task mode.
The guts of Lightning have been strengthened, too. CalDAV support and interoperability with CalDAV servers--the bits of the extension that send your calendar data back and forth--have been improved. Support for e-mail invitations, the iMip and iTip, has been improved, too. Although many of the major bugs that afflicted this feature were overcome in v0.8, 0.9 is even better. Also, memory consumption has been reduced, while various leaks have been plugged. Overall, the extension works better than it ever has at calendaring, and is more suitable as an Outlook replacement than it used to be .
For those who use the Lightning nightly builds, these changes are old hat. Except for one glitch that I experienced in August, the nightly builds have been impressively stable--at least for my needs.
You can read the Lightning release notes, and check out Sunbird for Windows and Mac, the stand-alone Mozilla calendar (if you don't use Thunderbird). It sports nearly identical features, except for e-mailing invites.