I found the most people using Classmates.com. A surprising number of people put something about themselves on there and you can email them. This does require that you know where they went to high school. Its also going to work better with people under 60.
The other one I use is switchboard.com for their family name in their old home town. If I dont get a hit in the area, I use the intellius free search at the bottom of the switchboard page with just the family name and look for relatives. The free intellius results gives names, ages, and allegedly related family members. Once you've found a relative, you can look up their phone number and address and snag them, get the number of the person you're looking for from them.
I said "allegedly" above, because my intellius record shows me as being related to a handful of people I've never heard of.
Linked-in and zoominfo also turn up some surprisingly good results at times.
Ancestry.com has a number of search tools that can be helpful. As morbid as it sounds, I remembered the names of some peoples elderly parents and found them (and their last known address) in the social security death records.
Using these tools, I've found pretty much everyone I ever looked for. Most of them have free trials and if you look around, promotional deals for a month or a year at pretty low cost. Just make sure you cancel three times before they autobill your credit card.
Most of my success came from finding one person and then networking with who they've kept in touch with. Linked-in is ridiculously good at this. I hooked up with a half dozen key people and their links and links to links to links produced all sorts of people.
I found one of my old high school girlfriends, and it turns out that one of her friends married one of my old friends, and he'd kept in touch with about 20 people I used to hang out with. I found another buddy that had gone to work with another old friend I'd been looking for, and he'd kept in touch with a handful of other people I'd always wondered about.
Now for the downside. I found a lot of people who ended up dead or sitting by themselves in some crappy apartment in a bad part of town. I found an old co-worker who became a financial advisor that was harder to shake off my leg than an amorous schnauzer. I also found a lot of immensely intelligent and talented people did absolutely nothing with their lives.
On the other hand, there were also some great outcomes.