Abandonia is an index of abandonware, “dedicated to classic DOS games”.
Another DOS focused site – the name gives it away – is DOS Museum.
One of the best sites to look for an ex-commercial video game is Wikipedia. That’s right – on one of their pages, they keep a pretty up-to-date list of abandonware (loose interpretation) titles.
Another site that takes on a wider array of (also non-DOS) games is Remain In Play.