There's also AROS, which also runs on standard pc hardware (as well as 680x0, ppc, arm, x86-64), and is free (MOS licensse is roughly 100 euro, give or take a little depending on which hardware you run it on. License is charged per machine, although a transfer is sometimes possible if you can prove youre transferring to a different machine for a reason). You can of course try MOS for free (for 1/2 hour, then system is slowed down drastically) if you have hardware that's supported.
All 3 options (MOS, OS4, AROS) have their pros and cons. The differences between all 3 are smaller than most people here will try to convince you of.
Also, I appologize in advance for the almost inevitable reflux action most people here will demonstrate when they get the smell of a potential new user of their chosen amiga clone system.
