The following are very old DOS text-mode games, and quite crude by today's standards. But they're fun and free, and even include QuickBASIC source. They're also potentially useful for very old or very small machines that don't support a graphical user interface. If nothing else, they're worth a small chuckle.
Concentration
A version of the classic card game that is easy and fun enough for children, but engrossing enough for adults. It will run on color or monochrome systems, and a mouse is recommended but not required. Includes QuickBASIC 4.5 source. Freeware.
Midnight Oil
Midnight Oil, otherwise known as La Belle Lucie, The Fan, Clover Leaf, and Alexander the Great, is a solitaire card game that is simple to play yet difficult to win, and extremely addictive. Midnight Oil will run on color or monochrome systems, and requires a mouse. Includes QuickBASIC 4.5 source. Freeware.
Poker Squares
A solitaire card game that's easy, fun to play, and competitive. The game requires no special hardware. It will run in mono or color, and will use a mouse if one is installed. Includes QuickBASIC source. Freeware.
Strategy
A solitaire card game that requires skill, concentration, and a good measure of luck. Strategy will run on color or monochrome systems, and requires a mouse. Includes QuickBASIC 4.5 source. Freeware.
All of these games are freeware, and include source code.
