Population Clock

Check out this webpage where US census Bureau estimates the world population real-time. Just wait for some time and refresh the page. You will see the increase in population in that few seconds you took to refresh the page.
The number I got on 08/25/07 at 01:50 GMT.

You can find some similar resources at:
1. http://math.berkeley.edu/~galen/popclk.html
2. http://www.ibiblio.org/lunarbin/worldpop
3. http://www.geocities.com/kalblando/popclock.htm
4.http://www.medindia.net/patients/calculators/worldPopulation.asp