Legendary Blade Runner designer Syd Mead helped to inspire the AT-ATs in Star Wars: The Empire Strikes Back, with some "walking tanks" he designed for a U.S. Steel catalog. A few years later, Mead was allowed to do his own walker design, when he created the first art for the power loader in Aliens.
Mead's original power loader concept art, posted over at Film Sketchr, actually looks more like the walkers in James Cameron's more recent film Avatar. Cameron ended up designing the power loader himself, and here's what his artwork looked like:
According to the Alien Stories blog, the climactic set piece where Ripley uses the power loader to confront the Queen was originally from Xenogenesis, the $20,000 short film Cameron had started when he was young and never completed. In Xenogenesis, "the two protagonists, a man and a woman, are hounded by a gigantic robot. The woman manages to flee, but the other is forced over a precipice and hangs perilously over a chasm. The robot leans in to finish him, but a far-off wall panel is forced open, revealing the woman – now encased in a robotic vehicle and ready to do battle for her partner's life."
There's also a great quote from Cameron about why he wanted to have Ripley use a power loader to fight the Queen, instead of a gun:
I wanted to have the final confrontation with the Alien be a hand-to-hand fight... To be a very intense, personal thing, not done with guns, which are a remote way of killing. Also, guns carry a lot of other connotations as well. But to really go one on one with the creature was my goal. It made sense that Ripley could win if she could equalise the odds. So there had to be some way of amplifying her strength, in a way that was not a comic-bookish sort of concept, like taking a pill.
At a certain point, I was toying with the idea of having the Marines have battle suits. But then I thought, 'oh, no, you're going to see that coming a mile away'. Anyway, how would Ripley know how to operate a battle suit? They wouldn't be teaching her. It was really critical to the story that she emerge under pressure as the person who really takes control. They discredit her at the beginning; the last thing they'd do is hand her a gun and teach her how to use a battle suit.
Tons more details, including a really, really weird prank the VFX team pulled on Sigourney Weaver when she was strapped into the power loader, over at Alien Stories.