All this plugin does is automatically run "/who" on the console every 10 minutes. This gives you a running log of who is on your server at a regular interval. I personally use the great WhosThere plugin so I get nice colorized output, but it doesn't matter what plugin you use; this plugin just runs "/who" and the resulting output is captured in your console log (usually server.log).
I wrote this because I used to have this feature via various other plugins (such as McMyAdmin), but since I've gone away from those plugins, I no longer had this capability. I've been using it on my server for about a month now and just noticed there's nothing similar on dev.bukkit so thought I'd release it in case there are others out there who want this feature. It's a very simple super lightweight plugin, the heart of it is only 1 line of code.