Two years ago, I posted about inspot.org, a site you can use to inform partners about their risk of having an STI. For example, if you have herpes and want to warn an ex, but are too embarrassed to contact him/her directly, inspot.org will send that person an anonymous email telling they may be at risk and where they can get tested.
I think this site is great as it promotes positive behavior - warning someone that they may be at risk, or even putting their future partners at risk - but also allows anonymity, which is very important in the case of STIs.
InSpot.org: En Français
InSpot.org: En Español