Uncle Sam

DC Comics Uncle Sam

Didn't think Captain America was patriotic enough? Never fear! Uncle Sam is here!
Quality Comics
Uncle Sam was originally published by Quality Comics. He was the spirit of a solider killed in the American Revolutionary War. He appears whenever America needs him. He had a costume-less sidekick Buddy Smith. He appeared from 1940 to 1944.
DC Comics
DC Comics bought Quality Comics' characters. This included Uncle Sam. As such, the company used the character. In the 1970s, he turned into the leader of the Freedom Fighters, a team of former Quality Comics characters that lived in alternate reality Earth-X.
Uncle Sam's origin was changed. He is a spirit created by the Founding Fathers via occult rituals. It takes various psychical forms via possessing people. In 1870, the spirit became Uncle Sam and possessed the body of a dead cartoonist. He fought in World War I and II (using two different hosts). In Infinite Crisis, the Freedom Fighters were either killed or injured. It is unclear if Uncle Sam survived or not. When the bodies are found, Uncle Sam was missing.
Uncle Sam reappeared in Uncle Sam and the Freedom Fighters. He reappeared in the Mississippi River. Sam took control of S.H.A.D.E.'s Freedom Fighters. Here Sam's hatred of lethal combat was emphasized. Uncle Sam had minor roles in Final Crisis and Blackest Night. In the New 52 series Human Bomb, an African American version of Uncle Sam appeared. He told by a S.H.A.D.E. leader to capture the titular character.
Powers And Abilities
Uncle Sam has variety of powers. These include super strength, size-alteration, invulnerability, super speed and minor clairvoyance. He had the ability to travel to parallel universes. However, he was shown unable to use this ability later on. Uncle Sam has the ability to transport objects (including himself and other people) to the Heartland, a pocket dimension.
References:
http://en.wikipedia.org/wiki/Uncle_Sam_(comics)