http://mathoverflow.net/questions/23175/mathematically-mature-way-to-think-about-mayervietoris
http://mathoverflow.net/questions/97621/mayer-vietoris-implies-excision
http://mathoverflow.net/questions/108819/is-there-a-mayer-vietoris-spectral-sequence-of-motivic-cohomology-for-closed-cove
nLab page on Mayer-Vietoris