AROS is pretty cool and free, but lacking many key Amiga elements under the hood.
MorphOS felt very next gen Amiga to me, and it just had a price cut. It's kind of hard to fully explore within the time frame of the 30 minute demo, but once you really dive into it and throw some classic apps at it, I think you'll be pretty amazed. Though just make sure your Mac has more than 32Mb of GFX RAM or else you'll find yourself restricted in what all you can do.
I have heard OS 4 is coming along well, but is not yet as polished, stable, or cheap to run as MorphOS. Though I've never touched it so I'm not certain as to how good it is, but it does run classic apps like MorphOS so that has to count for something.
Last but not least there is Amithlon. It's tricky to get going, but when you do it pays off...
Very fast like MorohOS in regards to running classic apps.
