This page will only work with a graphical browser implementing Javascript 3.0 or higher. It fairly accurately clocks the current population and reflects its growth using a second-order approximation of the exponential; in addition, if you enter a date in any form Javascript will accept and hit the "Do this time" button, it will use its magic formula to estimate the population for any date from 1900 to 2100, although the estimates become unreliable outside the range 1950 to 2050. Use the "Un/Freeze" button to "freeze" the clock at a certain time while it is ticking, or to resume it ticking with the present population after you have frozen it or asked for an estimate.
Those familiar with the earlier version will note that the range in which the new version gives reasonable estimates has increased considerably. It is of some interest to note that this approximation predicts that the population will start decreasing before the end of the 21st century.
The raw data is from the U.S. Census Bureau, and is not uncontroversial. For instance, the UN, with its data, considered 1999 October 12 to be the "day of six billion", but the approximation here (which I for one have more faith in) puts it at 1999 June 18 or 19 (depending on what time zone you're in), nearly four months earlier.