You don't see a lot of that on the Mac, mostly because of the culture. A lot of the big players in the Mac development world have been developing software since the 90s or earlier, so have a tangible sense of quality about their work. Newer developers are usually attracted to developing Mac software for that reason, and they don't want to disrupt it.
It's becoming increasingly more popular for developers to release a free, ad-laden version in addition to the paid version though. NetNewsWire is one example and Kiwi is another.