Former Flag of Belarus, Used in 1918-1919 during the Belarusian People's Republic, in the second world war by the German occupied Belarus, Also used by the Belarusian Popular Front in the Perestroika Period, and as the flag of Belarus from 1991-1995. Nowadays, this flag is Popular with those who oppose the current government of Belarus, the Rada of the Belarusian Democratic Republic, which is the world's longest government in exile.
Note: I don't personally support the two sides, and this flag is purely for historical purposes.