This is a great side-scroller developed by ID software and Apogee. There are six games divided into two series. The second series has better graphics than the first, but they are all great games.
Note: I am not sure if it is completely legal to download the second and third games of each series (Keens 3+4 and 5+6) So I am linking you to someone else's page. It is legal to download Keens 1 and 3. Enjoy!