Streaky the Supercat is a Fictional character published by DC Comics. Streaky first appeared in Action Comics #261 in February 1960, and was created by Otto Binder and Jim Mooney.
One of a series of super-powered animals (which also included Krypto the Superdog, Comet (DC Comics) the Superhorse, and Beppo (comics) the Supermonkey) that was popular in DC's comics of the 1960, Streaky was Supergirl's pet Cat that was given super-powers by an unusual form of Kryptonite.
I love Streaky!
Supergirl was experimenting on a piece of green kryptonite in an attempt to find a way to neutralize its deadly effects. When her experiment failed, she tossed the kryptonite (or "X-Kryptonite", as it became known as) out the window. Though her experiment failed, her pet Streaky came across the X-Kryptonite, and was exposed to its radiation. As the comics described it: "Due to the unique combination of chemicals in X-Kryptonite, it has given Streaky superpowers!"
Streaky's powers consisted of a smaller subset of those of Superman and Supergirl, including flight, super-strength, super-vision, and super-speed. Streaky made sporadic appearances in comic stories through the 1960, and even became a member of the Legion of Super-Pets, a group consisting of the superpowered animals mentioned above.
Streaky had a 30th century descendant named Whizzy (who also had superpowers).
Streaky's last pre-Crisis on Infinite Earths appearance was in June 1970's Adventure Comics #394. Presumably, when all of Earth's kryptonite was transformed into iron in Superman #233 in 1971, Streaky's X-Kryptonite power source was eliminated as well, ending his super-career and returning him to the life of a normal housecat again.
During Grant Morrison's run in Animal Man, an alternate, grey-furred Streaky (which, however, left an orange contrail with the telltale lightning bolt marking) made a brief appearance as one of the pre-Crisis characters that were returned to reality by the Psycho-Pirate's medusa mask. This alternate Streaky attacked "Overman," an evil parallel universe Superman with his heat vision before being hurled out of the asylum. Presumably, this Streaky vanished from existence when the Psycho-Pirate's episode of madness ended, along with his human counterparts.
Various cats inspired by (and sometimes named) Streaky have made appearances in the Crisis on Infinite Earths DC Universe, but none so far have acquired superpowers.
At the 2007 New York Comic Con, when asked if Streaky would be returning, Paul Dini said that the “cat’s out of the bag”. 
In Supergirl #10, it is revealed Kara has a female pet cat. In #14, it's revealed that the cat's name is Streaky because "she doesn't get the concept of the litter box."
In other mediaRectify
One scene of the Batman: The Animated Series episode "Deep Freeze" had a group of robotic toys which all resembled classic comic book characters, Bat-Mite, Mr. Mxyzptlk and Krypto and one of the toys resembled Streaky.
In Superman and Justice League, Streaky has made a few appearances as an ordinary housecat of the Kents. As an injoke in the episode "Mxyzpixilated", Mr. Mxyzptlk demonstrated his powers by making Streaky fly around the room.
In Justice League episode "Comfort and Joy" J'onn J'onzz does not feel the Christmas spirit until he wanders around Smallville (DC Comics). At the end of the episode he is seen in true Martian form, petting Streaky while singing.
In 2005, Streaky became a supporting character on Cartoon Network's Animated series Krypto the Superdog. In this series, Streaky once again has superpowers, and is the pampered pet of Krypto's owner's next-door neighbor, Andrea. In the series Streaky goes into a lab, where he crashes into a duplicator laser. The laser shoots Krypto, which bounces off him and into Streaky, giving the cat a portion of Krypto's powers. Unlike the original comics version, who was immune to the effects of Kryptonite radiation, this Streaky does share Krypto's vulnerability to green and red Kryptonite.
Streaky also appears in the Legion of Super Heroes (TV series) animated series. He appears in the episode "Message in a Bottle." This version of the character is a native to the shrunken city of Kandor. When Brainiac 5 turned the Kandorian sun from red to yellow, Streaky gets his superpowers.