It's been a long time since development of Kerbal Space Program began. 5 years ago Felipe started work on a 2.5D game that would eventually grow to the game that we all know and love. It's been quite a ride from the first, careful steps to a full fledged rocketry game that has inspired many people around the world. Those of you who have been around for a long time will know that Kerbal Space Program started out as freeware. Versions 0.7.x up to 0.13.x were free for those brave enough to try them.
Back in 2012 when I was a new member of this community I collected these versions of KSP and uploaded them for others to enjoy. People brought in more versions of the game that had then already seemed lost to eternity. Almost all free versions of the game were eventually brought together and offered for download. After the forum breakage in 2013 @xorg was awesome enough to restore the post and provide new links for all these 'community sourced' old and free versions of KSP, and I'd like to give a him a special mention here for that.
Now it's 2016 (time flies when piloting rockets) and unfortunately the community sourced downloads for these old versions were recently made unavailable1. I wanted to make these versions of Kerbal Space Program available to download again, but using the old files which had not come from Squad directly would not be wise. With the great help of @HarvesteR, @aLeXmOrA and @Chuchito we're now able to once again offer these versions for download, from an official source no less!
It's always been great revisiting the past to play around with a version of KSP from before it had such fancy tools as tank stacking, symmetry mode in the VAB or even a persistent save file. I'd encourage you to give it a try!
As for the legal notice: these builds of Kerbal Space Program are offered "as is" without any warranty and with the express warning that we cannot accept any liability for damages resulting from their use (though, they should work just fine). Install and use at your own risk.
If you're interested in what exactly you'll find the KSP Wiki has an overview of the version history. Download links are below.