OK, here we go:

Revolutionary War:
Washington: General & Commander of the Continental Army
Monroe: Continental Army lieutenant colonel

Indian Wars:
Wm. Harrison: ensign in the First Infantry of the Regular Army, rose to vice captaincy
Jackson: General in Creek and Seminole Indian Wars as well as War of 1812
Lincoln: captain in the Black Hawk War

Mexican War:
Taylor: General
Pierce: brigadier general

Civil War:
Grant: General-in-Chief of the Union Forces
Garfield: major general of volunteers.
Hayes: brevet major general
McKinley: Enlisted as a private in the Union Army, mustered out at the end of the war as a brevet major
B. Harrison: Colonel of the 70th Volunteer Infantry
Arthur: Quartermaster General of the State of New York

Spanish-American War:
T. Roosevelt: lieutenant colonel of the Rough Rider Regiment

World War I
Truman: captain in the Field Artillery

World War II
Eisenhower: General, Commander of the Allied Forces
Kennedy: Navy lieutenant & commander of a PT boat
L. Johnson: Navy lieutenant commander
Nixon: Navy lieutenant commander
Ford: Navy lieutenant commander
G.H.W. Bush: Navy lieutenant pilot

Carter: Navy lieutenant & senior officer on one of the first nuclear submarines (1946-1953) Carter is the only president to serve in the military without being involved in combat situations, though he attended the Naval Academy during WWII.

So, 8 president were generals. Interestingly, all recent presidents which served, did so in the Navy (Eisenhower being the last Army man). Of course, our current pretender was in the Air National Guard.