The flag of the United Kingdom (3:5 version).  This flag is the Union Flag in the 3:5 ratio exclusively used on land. At sea, the correct ratio is 1:2.
Design data from http://fotw.vexillum.com/flags/gb.html; original code is from Image:Flag of the United Kingdom.svg
Original code by Stefan-Xp with modifications to ratio by Yaddah.