The early 1980s saw the emergence of a new breed of software developers who gave their games away for free (usually through dial-up BBSs) in the hope that some players would enjoy the games enough to "register" them--usually by sending money over good old snail mail.
Not many players did, but that didn't stop the game makers. Eventually, the shareware method became more refined, offering a free episode as a trial with the promise of more episodes upon payment. The golden age of PC shareware that this model inspired lasted less than a decade, but it left us with thousands of memorable titles that changed the modern gaming landscape. From those many great games, I've chosen a dozen that I think are particularly innovative, influential, and just plain fun.
Many of these games are still available for download or purchase online today. In almost all cases, you'll need DOSBox, an MS-DOS emulation program, to run the games correctly in Windows, Linux, or OS X.