Osamu Shimomura, Martin Chalfie, Roger Y. Tsien.All of them are Americans and will receive 1/3 of the prize. The green fluorescent protein may be found e.g. in sea pansy. It is used e.g. in fluorescent microscopy - you observe green light directly created by a (marked) cell out of the incoming blue light rather than reflected light.
This video explains GFP well. Thomson Reuters (TRF) had fluorescent proteins among its candidates but it expected one winner only, Tsien.