If you are looking for alien stencils to use for carving your Halloween pumpkin, you can find a couple online for free and lots that can be printed out for a fee.
Free
DLTK 1: Free printable stencil of an alien with very pointy ears. It can be printed out from the website DLTK-Holidays.
eHow: Classic design of an alien creature with big eyes. This pumpkin carving pattern can be printed out for free from the eHow website.
To Buy
Venom (Face): Venom is an alien villain in the Spider-Man series.
Alf: Alf is the furry creature who appeared on the TV show of the same name during the late 1980s.
Alien: The scary creature from the sci-fi horror film which was first released in 1979.
Gallaxhar: Gallaxhar is an Martian from the animated Dreamworks movie Monsters vs. Aliens. Gallaxhar was voiced by actor Rainn Wilson.
Predator: Predator is an extra terrestrial warrior from the sci-fi action film.
The Visitor: A classic image of the visitor from outer space.
Egg: A creepy extra-terrestrial creature sitting inside of an egg.
The Great Gazoo: The Flintstones Martian sitting with his arms behinds his head and legs crossed.
Invader Zim: Zim from the Nickelodeon TV show.
Toy Story Alien: Stencil of the cute three-eyed toy.
Mars Attacks!: Carving patterns of the Martians from the Tim Burton comedy.
Aged Alien: A wrinkled extra-terrestrial being.
Roswell Grey: Unique stencil of a Roswell being.
ET: The most lovable extra-terrestrial of all-time.