M/M (Paris) is, unsurprisingly, an art and design partnership working out of Paris, established in 1992 by Mathias Augustnyak and Michael Amzalag. Most of their famous work is in art direction and collaborations with other artistic figures, including musicians, fashion designers, and magazines. They got their start designing record sleeves, and moved up quickly from there.
Personally, I like their strong use of black and white, frequently complemented with splashes of color, as well as their mix of digital-looking figures and pixels with more hand-drawn typography or imagery. Their work has a fluid quality that appeals to me.