This site tries to gather open-source reimplementations of great old games in one place. If you think that some information is missing or have any suggestions - send a pull request via Github or a mail to me.
Since all these projects are open-source you can help them and make this world a better place. Or at least you can play something to appreciate the effort people put in them.
Similar resources: check out Liberated Games, which specializes on commercial games, which were made free, and Free Gamer, a blog dedicated to open source game news.
All games that were added in last month are highlighted like this.