Kinda disappointing that nobody have mentioned Parkitect yet. It can best be described as a modernized RCT2, lacking a few small features but improving in so many others. It has its own thread in this section, and I believe it runs like a charm on Mac too.
I know it's still technically in development, but the game is mostly complete by now, lacking only a campaign mode which is set to be introduced some time this autumn. In the meantime, new rides and/or scenery is introduced every month. Follow its development in weekly posts at http://themeparkitect.tumblr.com/