These sites are great for tracking down long lost friends. You can access Birth records, Electoral role data and social networks from here.
192.com allows search of electoral role data. If you are clever, you can get the data out of the engine without paying for searches.
Ancestry.co.uk have free BMD Indexes (Birth Marriage Death). Records post 1984 can be searched free of charge, if you are clever.
Facebook.com Great social networking site. Keep track of all your friends
Myspace Another great social networking site
Friends reunited. Has Workplace and School groups which are ideal for finding old friends. Free of charge.