The official site.

an online store selling new and used computer and video games.

search engine that ranks by popularity

an archive of Usenet newsgroups; useful for finding discussions; formerly DejaNews

a site specializing in trading computer games; useful if you want to trade one of your games for Mazes of Fate.

Many older games listed on MobyGames are out of print. Auction sites like eBay are the best place to find them.