Slicks 'n' Slide is the only shareware program that I have registered... because it was easy and cheap and, well, it rocks. =)
Slicks is a racing game for MS-DOS, written by Timo Kauppinen. The game is shown from top-down perspective. Up to four tiny little cars go around the track. And, of course, burn rubber and leave lots of skidmarks. After the race, it really looks like as if something had happened!
The later versions added animated terrain features (such as waving flags of Finland, and piles of tires that spread when hit by cars), lots of different vehicles (cars, tanks, motorcycles, trucks, and so on), music, and even weapons (that fortunately could be turned off!)
Registered versions got a huuuuuuuge load of tracks with the game, and also got the track editor to make new tracks with! (Too bad making the control points for the AI was pretty difficult...)
All in all, this game had its good moments, and if you can get it running on modern machines, I can definitely recommend it!
At the moment, the home page appears to be at <http://slixnslide.tripod.com/>. The latest (last?) version is 1.30d.