Facebook | What do you want to advertise?
I thought I'd try Tim Davies' idea and use Facebook ads to get an estimate of users in Derby, UK.
Turns out to be around 26% of the population (based on a 2004 estimate).
Another tweak, and I found I could reach 300 people whose language is Polish. Powerful stuff.
Go here: www.facebook.com/advertising/ and click Create an Ad.
Give any web address.
Type anything in the boxes to name and describe your ad.
Play around with your target area/demographics and watch the estimated number of users change!