Below is an illustration of b-matching from (Huang,Jebara AISTATS 2007) paper. You start with a weighted graph and the goal is to connect each v to k u's to minimize total edge cost. If v's represent labelled datapoints, u's unlabeled and weights correspond to distances, this works as a robust version of kNN classifier (k=2 in the picture) because it prevents any datapoint from exhibiting too much influence.
They show that this restriction significantly improves robustness to changes in distribution between training and test set. See Figure 7 in that paper for an example with MNIST digits.
This is just one of a series of intriguing papers on matchings that came out of Tony Jebara's lab, there's a nice overview on his page that ties them together.