These formulae duplicate software provided by OS X, though may provide more recent or bugfix versions. They may also provide conflicting formulae that I consider to be more desirable, i.e. the latest hpn-ssh enabled version of openssh instead of the absolute latest.
Just brew tap ccosby/ninja and then brew install <formula>.
If the formula conflicts with one from mxcl/master or another tap, you can brew install ccosby/ninja/<formula>.
You can also install via URL:
brew install https://raw.github.com/ccosby/homebrew-ninja/master/<formula>.rb
brew help, man brew, or the Homebrew wiki.