If you're a fan of science fiction, you've likely heard the complaints about the alien races in Star Trek and Star Wars: they're only humans with bumps on their faces, pointy ears, or green skin. So many stories that have used this kind of alien race have leaned on the crutch that is a common ancestor race, but doing so simply enables authors to avoid being creative. I can't help but feel that aliens in science fiction just need to be stranger and more... alien.
To give you an idea of how strange aliens should really be, take a step back for a moment and consider explaining a few things about humans to a creature from another world. Keep in mind that this alien may not understand emotions, may not communicate via sound or see using what we consider visible light, and may not respirate. How would you explain laughter or crying? How would you explain emotion-triggered goosebumps? How about music? Religion? You could do it of course, but the amount of background necessary for a decent explaination of any of those is significant. And even after a good explanation, it's likely that these things would still seem bizarre to an alien. That said, how many analogous things might an alien race have that seem insanely counterintuitive? Things that people could never have imagined? Things that might seem immoral, insane, or backwards to us? It's entirely possible than an alien species' entire nature might be so offensive to us that war would seem the only natural option. Consider alien races such as those depicted in The Blob, Starship Troopers or The Thing. Can you imagine negotiation or peaceful coexistence?
Scott Sigler has done an excellent job of creating alien races that are more than simply human variants. The Sklorno are clear-skinned grasshopper-like beings with tentacles and eyestalks. The males are a tiny fraction the size of females, cannibalism is accepted as a normal part of daily life, and they worship certain living creatures as dieties. The Harrah look like flying sting rays, "see" via sonar, and can live on gas giant planets. Even better is a living organism described in part of Sigler's horror series "The Crypt" which is a fast-growing lattice of "sticks" that hunts by expanding and piercing the bodies of prey. Yeah - it's a horror story. But the alien there is pretty darn original.
Nor Crystal Tears is a book by Alan Dean Foster which describes first contact with a mantis-like alien race through the perspective of the aliens. And in Michael Crichton's The Andromeda Strain, the alien is microbiotic in nature. Aliens like this one, like The Blob, and like Scott Sigler's sticks have to be approached differently because they're not very intelligent, perhaps not even sentient. But they're interesting and creative.
I recently finished reading Starswarm, a novel in which the alien intelligence is aquatic plant life, and is closer to a computer AI than it is to a human. This is the kind of creatively written alien I'd like to see more in science fiction.