Already mentioned in the literature section, but (because it bears repeating) Reepicheep from the The Chronicles of Narnia films. Modeled after Errol Flynn and voiced by Simon Pegg, Reepicheep goes mouse-to-man several times... and wins! Even though he is as cute as Lucy says (he prefers "courageous, courteous, or chivalrous"), he's a force to be reckoned with. He even tried to take on an army of Telmarines by himself! He easily manages to be the best part of both of his movies.
In battle, Quorra of TRON: Legacy knows how to kick ass with her kickboxing and dogfighting abilities. When she's not fighting however, she's naive and has a childlike-innocent view of the world.
Yoda, probably the most iconic example of an old Badass Adorable ever. Short height, big cute eyes and his wisdom knows no limits. And yet, one of the most powerful Force users and sword fighters of the Star Wars universe.
Anakin Skywalker in The Phantom Menace. Born of the Immaculate Conception, incredibly sensitive to the Force, a mechanical genius on his spare time and one of the best podracer pilots in the galaxy, not to mention his instant mastery when it comes to pilot a Naboo royal starfighter, destroying the Trade Federation's mothership thus putting an end to the conflict on Naboo in the process. And he was only 9 years old.
The Ewoks. Cute fluffy teddy bears in hoods. Fierce warriors (with teeth) willing to take on the Stormtroopers with nothing more then slings and rocks. They kick some serious rear.
Leeloo from The Fifth Element: cute, innocent and perfectly capable of breaking every bone in your body without breaking a sweat.
Steve Rogers of Captain America: The First Avenger is a sweet-tempered, well-mannered and kind-hearted boy-next-door who just happens to possess a body rebuilt to human perfection.
From Master and Commander, Midshipman Blakeney. He's this utterly adorable curly-haired twelve-year-old, who... gets his right arm amputatedbut doesn't let it slow him down, becomes something of a "fighting naturalist," and leads a group of men more than twice his age into battle, thus helping secure a decisive victory over the HMS Surprise's archenemy, the Acheron. His inspiration is Lord Nelson, also missing an arm.
Bumblebee in the Transformers Film Series. Bright yellow, mentally younger than most of the other Autobots despite being physically around the same age, and over the course of the trilogy; racks up a Decepticon body count second only to Optimus Prime himself. In Transformers: Dark of the Moon, he even gets more weapons and armor to symbolize his evolution from guardian to full-fledged warrior. Yet, he still has his blue eyes and still talks through his radio; never once losing his connection with the younger audience of the movies.
Mako Mori in Pacific Rim, a cute Japanese survivor who wants nothing more than to have the heads of each and every Kaiju on a pike.
Kate (Mary Elizabeth Winstead) from the prequel to The Thing (2011). Intelligence and bravery aside, she has been described by a review as "looking like a cute teenager trying to buy booze with a fake ID rather than an actual adult."