Graph Your Inbox uncovers your Gmail account's hidden stats
Just fire up this Chrome extension, enter some search terms (boolean searches and Gmail advanced searches work, too), and away you go. Figuring out how many Twitter followers or Facebook emails you've received over time is just one possibility. You could search for a friend's email address and get an instant graph of your relationship (your email relationship, anyway), or you could see if you've been getting more emails from work lately.
Graph Your Inbox is a powerful tool that gives you a new way of looking at your Gmail account, and it's a lot of fun. It's a bummer that it only works with Chrome right now, though.