If you have an old laptop or desktop that is almost dying, or you just wanna replace Windows by a good Operating System then I recommend you Elementary OS!
As you know, most of the computer users run windows. And all the ones that run it have problems. Viruses, and a lot of problems that I used to have in the past before switching to Linux.
Elementary OS is a Linux distro (Distribution) based on Ubuntu. It is known to be fast, and very convenient for people that have old laptops, or just wanna start using Linux.
The thing that I like on that distro that it offers built in applications that you will actually use. It also mentions on its site:
Apps You Need, Without Ones You Don't.
Elementary OS is open source
You can have access to its source code here on Github! Elementary OS
It's Free!
As other Distros, it is free! But you can also pay for it if you want!
It's Safe!
You won't have problems while installing apps, and no one will spy on you. Trust me :)