Another SQL script I thought would be interesting and a good one to quote to potential advertisers.
The total number of confirmed new registrations in the last 30 days and the average per day.
SELECT COUNT(userid) as Total_registered_users, COUNT(userid)/30 as Average_per_day from user WHERE user.usergroupid = 2 AND user.joindate >= UNIX_TIMESTAMP(DATE_SUB(NOW(),INTERVAL 30 DAY));

Of course you can change the timespan from 30 days to 60 or whatever by changing the 2 occurences of '30' to however many days you want.
And by checking that the users are in usergroup 2, we are only looking at those members who activated their registration.

Hope this is useful to the non-SQL oriented folks out there.