A few weeks back the sim card to my cell phone died. I've been with AT&T (was Cingular, and Pac Bell) for the last 10 years, and lost over 250 accumulated phone numbers. For many people it was my only form of contact, including my own parents. Fortunately I was able to track down many of the important numbers through Zabasearch.com
Zabasearch.com is a great search engine for finding long lost friends. It is surprisingly simple to use, and amazingly accurate. Another search engine I use is Whitepages. If you have lost contact with someone over the years, try using it. If you only keep your friends contact info in your cell phone, learn from my mistake and backup their numbers. Friends are an important part of life, and sometimes reaching out to a long lost friend can really brighten someones day. If you lose that number, you lose that opportunity.