Registration Statistics by tombenner is a WordPress plugin that adds a tool in the WordPress admin section that allows administrators to choose a date range and see graphs and tables showing the number of registrations (i.e. new users) that occurred on each day and the number of cumulative registrations on that day (the total number of users who had registered on or before that day).
The graphs and tables are interactive; you can sort the tables by date or by count, and you can mouse over the graph to see the count at that point. You can also click on a point on a graph and choose to view a list of posts that were published on that day or during that month, which may allow you to see what posts might’ve caused a peak in registrations (and thus gauge what kinds of posts you can make in the future to spur on more registrations).
Source Code: https://github.com/tombenner/registration-statistics
Download and info: http://wordpress.org/extend/plugins/registration-statistics/