A great set of scripts by by Iván Rodríguez Murillo that builds the Structure from Motion solution VisualSfm as a self contained App on OSX. Just clone the repository from GitHub and execute “run ./scripts/all.” You might need to install XQuartz from http://xquartz.macosforge.org
Libccv is an open source image classifier and computer vision library.
It runs on Mac OSX, Linux, FreeBSD, Windows, iPhone, iPad, Android, Raspberry Pi. In fact, anything that has a proper C compiler probably can run ccv. The majority (with notable exception of convolutional networks, which requires a BLAS library) of ccv will just work with no compilation flags or dependencies.
Among lots of other things it has a SIFT implementation
The sources are available on github.