Maximally Stable Extremal Regions (MSER) is an algorithm of extracting affinely-invariant stable subsets of extremal regions, and can be used to establish correspondences between two images. Two implementations could be found @
http://vision.ucla.edu/~vedaldi/code/mser/mser.html
http://www.robots.ox.ac.uk/~vgg/research/affine/detectors.html#binaries