Some players have decided that playing old versions of KSP is what is known as "fun" and not "sadism". The oldest and hardest version is undoubtedly 0.7.3. But the rewards are thus much greater. There is also more explosions.
Old versions of KSP can be found here. All versions before 0.13.3 are free and can be downloaded legally. Versions 0.0 through 0.7.2 are not available, existing only in the hard drives of Squad computers.
To successfully play the oldest released version of KSP: 0.7.3; you must know a few things. The most important fact is that fuel tanks do not stack their fuel flow, there is no symmetry, and the atmosphere ends at about 37 kilometers. If you remember these, and you don't want the rest of your discovery experience ruined, stop reading here.