Didn't get your period and wondering exactly what time in your cycle that ex-sex escapade occurred?
If memory escapes you, fall back on MyBlackBook, "The Internet's First Secure and Confidential Online Sexual History Tracker."
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MyBlackBook's online ledger keeps track of all your sexual partners and the acts you performed together -- data that could either be smugly self-satisfying or just plain depressing. You can even examine how busy you've been math class-style: "activity" graphs, "hourly" graphs (to see what times of day you're most randy) and "position" pie charts!
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The site touts itself as helpful for users who've contracted an STD and need to figure out who spread it -- and who its been spread to. MyBlackBook also offers a VDNote service that computes the statistical likelihood that you've contracted an STD and compares you info with the site average (it claims to have almost 7,500 members) and the national average.
MyBlackbook's got a competitor in Bedposted, which is still in beta and has been getting so-so reviews. For now, we're sticking with what we know.