Chrome: Facebook, Twitter and LinkedIn are great ways for you find out more about a person, especially if they don't know them but are likely to meet them. FullContact is a Chrome plugin that shows the social profiles of the person you're exchanging emails with.
Not only does FullContact collect information from major networks like the ones we mentioned, it also supports Pinterest, Klout, Etsy and a few others.
FullContact also enables you to make notes and tag people. The only caveat with this feature is after 5000 contacts, you'll have to upgrade to a premium account.
Even though FullContact is extremely powerful, I found that it didn't always work as I had expected it to when matching social profiles. For example, when I was exchanging emails with a friend at her company email address, it pulled up her company's Twitter profile instead of her personal one.
Although FullContact doesn't work in Gmail's new pop-up compose window, it does work when you're replying to an email. You can also type in additional people's email addresses when you're replying -- which means you can still guess people's email addresses with it (like you did with Rapportive).