Best Calendar Add-on for Thunderbird. Rated 5 out of 5 stars
Good system, allow my to reply to events for Outlook and Google. Works good for my single calendar events, not sure about recurring. :) There are a few quirks, but its a work in progress and it's free.
I'm using the nightly build with Thunderbird Beta 3.03b, which can be found here (the Google provider is also available there):