Yes, you read it right. Mike On Ads is an cool website which analyses your web history and tries to guess your gender by using clever javascript. In the authors own words

"I modified the SocialHistory JS so that it polled the browser to find out which of the Quantcast top 10k sites were visited. I then apply the ratio of male to female users for each site and with some basic math determine a guestimate of your gender. The math is really quite simple, I just take:

1 / (1 + r_1 * r_2 * … * r_n)

where p_i is the ratio of men-to-women for the specific site. For example, if you had been to two sites that had a 2-1 ratio of men to women, the probability of you being female would be:

1 / (1 + 2 * 2) = 1/5 = 20%"

I gave the test and it turned out to be 100% accurate.

You have to click on a button (takes some time to load due to all those comments) and then after few seconds your guessed gender is displayed.

Links:

http://www.mikeonads.com/2008/07/13/using-your-browser-url-history-estimate-gender/#comment-44227