If you like retro point-and-click adventure games, check out The Telwynium. Don't let the beautiful but primitive EGA-style graphics fool you, this one actually has a really strong plot. Non-comedic stories often fall flat in adventure games, but this one works really, really well, and the puzzles make sense in the context of that story. The only comparable classic adventure game I can think of is Loom.
The game is pay-what-you want, supports all three major desktop operating systems, and even comes with source code.