I came across a site recently called
spokeo.com that allows you to see your friend's activity across the web, simply by typing in your e-mail address and password. It then pulls your contacts from your e-mail contact list and displays everything they've done. From updating an Amazon.com wishlist to posting a new blog on Myspace, it's all tracked across 20 social networks. However, it will only pull what they've done using that specific e-mail address. So if your friend is writing nasty blog entries about you under a different e-mail address, you won't see it.
I haven't decided if this is the coolest thing ever, or just another way for people to keep tabs on their ex.
Let me know what you think.