It would be great if there was a plugin out there to track the growth of a server. Basically it would record the maximum players online every day (or hour) and export them to CSV (or XLS). These statistics would be very useful in evaluating different techniques to achieve server growth.
I'm actually thinking of developing this for my server. I was thinking it would save the player count in a MYSQL database or something like that. If others see a need for it, I'll make it public after finishing it.