Along with the BT tank, the T-26 formed the basis of the Soviet tank park before the start of WW2 and during its initial period. The T-26 was popular at one time but the weak armour and its low speed made it easy prey for the enemy, and quite often there was no radio in the tank. By 1938, it was virtually obsolete.
The T-26 was the most numerous tank of the Red and Finnish Armies by the start of the Great Patriotic War in 1941 and also of the Spanish Republic during the Spanish civil war. During the 1930s and 1940s, it was second in terms of numbers after the T-34. It was constructed on the basis of the British Vickers Mark ME or ‘6-ton tank’ and was taken into service in the USSR in 1931 (source: Russian Wikipedia).
In all, 281 T-26s were sent to Spain from 1936-1938. It was also used against the Japanese at Khalkin-Gol, and in the Soviet campaign against Poland in 1939.
Many were captured by the Germans during Barbarossa and used as ‘trophy’ tanks and called the Panzerkampfwagen 737.
Other users were:
Finland, as mentioned. 126 tanks, several dozen of which were only decommissioned in 1961!
Turkey-64 single turret and 2 two-turret versions.
Afghanistan- 2 of the first two-turret version.
Romania- captured 30 T-26s although only one is known to have been used.
Chinese (nationalist)- 82 T-26B model.
Slovakia-2 tanks one of which was displayed at the Exhibition of captured weapons in Bratislava.