One of the simplest resources that works well on MacOSX is the Markdown Quick Look plugin found at InkMark. After installing this on OS X 10.9+, you simply select a markdown file in Finder and tap the space bar. I am not certain if the results are correlated 100% with Github but they are pretty good and make for a tiny unobtrusive offline viewer only solution. Mashable also wrote an article on 78 different tools if you'd like other options.
