One way Google calculates popularity of a website is the number of links to the site. If someone is prepared to put a link on a site to another site, they view this as a vote of popularity.
Some companies were quick to act so they set up “link farms” where you could buy 200 links to your site. Google are a bit smarter than this, so they now take into account of over what period of time links to your site were created.
Hint: Avoid “buying links” and try to slowly build up links to your site.
Quality, older sites tend to build up these links slowly over time. Thus, building a quality site with content that doesn’t get outdated can increase in value overtime.
You can check link popularity here: Market Leap. Also, look for quality of the links, rather than quantity. A link from Microsoft will have a lot more value than a link from bobshomepage.com.
Make money hint: Find websites in your interest area, particularly older sites, with good content and little advertising. Check out their link popularity, if it is good make an offer on the site to the owner. There are probably 1000s of goldmines out there just waiting to be bought.