Mozilla Sunbird is a calendar project that's being designed as a sister program to Thunderbird and Firefox. It's now at the 0.5 version, and there is a plug-in that integrates Sunbird with Thunderbird, called Lightning. It allows you to not only have a calendar, but send and receive meeting requests as well.
I'm running Thunderbird 2 under Ubuntu Feisty Fawn (7.04) on the AMD64 architecture, and could not find a pre-compiled version of this that worked. So, I decided to give it a shot. It was pretty easy, and the result is a plug-in that works with the AMD64 version of Thunderbird! Since I had trouble finding it, I thought I would share it.
Since I have just started using it, I haven't wrung it out, or tested all the options. Use at your own risk, etc.
You can download the plug-in here (UPDATE: See this post.) Happy storming! ;)