Sparky Linux is a blazing-fast distro that can keep your older machines running for years
One of the many benefits of using Linux is that it doesn't suffer from the "planned obsolescence" that you'll find in other operating systems. With Linux, you could find a distribution capable of bringing computers back to life that you were certain couldn't handle the rigors of modern-day needs.
That's not to say you're going to pull that old Pentium 75 machine out of the basement, dust it off, and make it your daily computer (although you could probably find a distro that would run on it). What it does mean is that you're not going to have to worry that every year, when your operating system is upgraded, you'll get the warning that your computer needs to be tossed to the scrap heap instead of something faster, newer, and more expensive.