A fast and free alternative to Windows and OS X
However, there's another operating system you might want to know about. It's called Linux and it's been around for a long time. In fact, it runs a large chunk of the world's servers and embedded computers. Android is based on it, and OS X is based on an offshoot of Linux's predecessor. So, what does that have to do with you?
A few good things about Linux are that it's free, most of the software for it is free, and it's very secure. There are also dozens of versions around targeted toward certain audiences. Linux Mint is one we've talked about before that's a good Windows replacement.
However, Linux Mint still has a learning curve for new users, and the default version requires newer hardware to run well. For older hardware, or a more streamlined experience, you might try a Linux version called elementary OS.
Elementary OS is visually more similar to OS X than Windows, but it's simple enough for anyone to use. It's also very lightweight, which means it runs great on older hardware, even low-end hardware from the mid-2000s.