hedwards: Alien Carnage/Halloween Harry is an interesting one as 2/3 of the game was free from release.
Paradoks: Not exactly (as far as I know). The game has 4 episodes and only 2 were available for free as shareware. Furthermore the second episode became available a year after original release.
It should be also noted that Alien Carnage version is much more problematic compared to Halloween Harry.
Hmm, I thought it was 3 episode game, I guess that is a slightly worse ratio. Still, for the time, that game was great.
Anyways, Jetpack was awesome,
https://www.adeptsoftware.com/jetpack/ . I haven't had a chance to play my copy of the sequel, but it's pretty good too.
Also, Skyroads is a pretty fun game, albeit shorter.
http://www.bluemoon.ee/history/skyroads/index.html