One of my favorite features about Facebook is that people can specify their birthdays and Facebook will notify me when a friend's birthday is approaching. The problem is that I rarely ever log into Facebook, so I usually miss out on these notifications. Up until now, I have been attempting to enter friends' birthdays into Microsoft Entourage - it has a nice, pop-up birthday reminder. Recently, I learned about FBCal, a Facebook application that generates an iCal stream containing all of your friends' birthdays. You can then subscribe to that iCal stream from your calendar application of choice (unless, your calendar application of choice happens to be Meeting Maker, which doesn't support iCal!).
So, basically, you install the plug in, it gives you the URL to the stream it generates, and you subscribe to that stream in a calendar application. After that, you will see all of your friends' birthdays in your calendar application.
I was able to get this working with my Google Calendar and it has been a really nice addition for me. I did, however, have to wait several hours after I subscribed to the feed for the birthdays to appear. If you attempt to use this program, please note that results may not be instantaneous. So far, I have been unsuccessful in getting it to work with the Lightning calendar plug-in for Thunderbird. I could really use it to work with the latter, because I live in Thunderbird all day.
All in all, I really like the concept of the application. It feels like it could be a little smoother, but maybe that will happen in future releases.