If you had access to a PC in the 90s, you probably played Chip's Challenge. The puzzle game was included in a number of Microsoft Entertainment Pack bundles that were often included with early versions of Windows, and is thus burned into the collective memories of a whole generation of gamers. Now you can stoke the fires of nostalgia on Steam, where Chip's Challenge is now completely free.
You can head to Steam and download Chip's Challenge right now. This is the full original game from designer Chuck Sommerville, with 144 puzzles to complete. This is the port that launched in 2015 alongside Chip's Challenge 2 (another set of 200 levels available for $4.99 / £3.99 / €3.99), lightly updated to run on modern machines.
Chip's Challenge originally launched way back in 1989 for the launch of the Atari Lynx, but it's the later Windows release where Chip's Challenge burned its way into most of our collective memories.