Lately I've used [link|http://www.classmates.com|Classmates] to get ahold of some old friends. You can sign up for free and register your name to the various schools you've attended, as well as view other people who've registered. However, in order to get their contact info you'll have to pay. They've gotten better too, originally they only tracked a single high school, yet I went to two. Now they track ALL your schools.
I've googled a few friends, and have had lots of old friends tracked me down(they tend to google on SpiceWare).