The web offers many ways to find friends that you may not have talked to in a while, and keep up with friends you may otherwise rarely talk to.
Facebook is a "social networking" site—a website where lots of people get together to talk. Sites like Facebook have the advantage of many side features, like games, movie reviews, book reviews, and different sorts of messages.
With Facebook, you create a free account using your email address, and fill out as much information as you want. You then search for people you know who are already on Facebook (Facebook can also use your computer's address book to look for friends), and add them as a "friend." From then on, when you go to Facebook, all of your friends' activity (messages, movie reviews, book reviews, etc.) will appear on your main Facebook page.
LinkedIn is a lot like Facebook, but for working professionals. It's especially handy for job hunters.
Nevertheless, LinkedIn works a lot like Facebook. You create a free account, then enter as much information as you want. While members of Facebook sometimes prefer a certain amount of anonymity, LinkedIn works best if you enter as much career information as possible.
Classmates.com
While Facebook focuses on connecting friends, and LinkedIn on connecting professionals, Classmates focuses on—surprise!—connecting former classmates.
With Classmates, you first search for your high school or college, then create an account. With the free account, you can browse listings, but you can't contact anyone unless you purchase a paid account.