What is the term for that?
A game that used to cost money but is so old that it is now available for free (though not always open-sourced).
Also, does anyone know where I can find a list of such games?
Unless what you're wanting to do falls under copyright, trademarks, or patents, you're free to do whatever. Generally speaking, mechanics aren't protected. Names are. Art is (and that includes things like music and level design). You can make a "clone" in the sense that it's a very similar game, but you can't call your Tetris clone "Tris" and not expect some lawyers to get involved. Likewise I wouldn't make a pacman clone with the same board layout and look of the characters. But you could probably do something with nearly identical mechanics and a different skin.
The term is 'Freeware' (if the game was officially released for free by the company late in it's life), or 'public domain' if the copyright ran out. Copyrights last too long in the US, and no game has hit the 'automatic' lapse of copyright.
There is also the term 'Abandonware', which usually means "the game/software is old and no longer being sold or supported". Many people think that abandonware can be distributed legally, but that is not true. Someone still owns the copyright on those games, though in many cases the exact owner is a mystery even to the owner.
Wikipedia has a list of freeware games: http://en.wikipedia.org/wiki/List_of_freeware_video_games
The terminology you're asking about is "abandonware" (which is a combination of elements from the words "abandoned" and "software"), but free doesn't necessarily mean the copyright holder permitted it to be free (see my final paragraph about downloads below...).
For a list of abandonware game titles, there is one famous web site that provides a fairly extensive list which is also categorized for further convenience:
A note about downloads: There are a few vendors who are known for allowing the old copies to be distributed freely (and I'm not aware of any that are [intentionally] open source), but as far as I know this is so rare that you might as well assume that any abandoned game you can download likely involves a copyright violation even if the company has gone out of business (you'll need to consult legal expertise to find out how copyright works in your jurisdiction with regard to a particular title).