Friends Re-united is a website that allows you to find friends/family/work colleagues from your past (providing they have registered on the site themselves). This site has now been going for many years, although recently it has had to revamp itself as its popularity appears to be decreasing.
Once you register (this is now free), you have the option of creating your own profile with photo's and personal information on site. You can also start searching for and sending mail to those who you find.
Personally I find the layout of the site a little confusing, and it takes too long to dig up some of the information you want to find. Saying that, it is a useful way of tracking people down and making contact with them. Another benefit of this site is that it allows you to look others up, without necessarily having to create a profile of yourself. Perfect if you want to have a nosey at others, without revealing anything about yourself - though personally where is the fun in that!
Due to its decreasing popularity I find that people tend to join and create a profile, then do nothing else. Of course at any time you can update your profile, but not many people seem to do this - so often the information you read is out of date. Its a shame, but I think newer sites such as facebook seem to becoming more popular social networking sites.
Overall worth joining for an initial nosey, but unlikely to be a useful long term option.