Alien lifeforms, completely different from our own typical build can be the spice of your science fiction, or fantasy games. But how do you imagine lifeforms that you've never seen? Before you know it, everything starts to look either like insects, like Lovecraftian creatures, or like the Aliens from the Ripley-driven film sequels. This in itself isn't bad. But there are a few other possibilities.
Here's some sites I stumbled upon that may give you entirely new ideas. You could see them as a jumpstart to Xenobiology 101.
The first is Furahan Biology blog, run by a Dutch scientist. It features quite a few nice thoughts on xenobiology and the imaginary planet Furaha. Similar blogs are The Xenobiology of Nereus, run by Evan Black from Utah, Xenoblog by the mysterious lady Amoeboid, and Biology in Science Fiction by Peggy Kolm.
The second thing to look at would be the Aldebaran, Betelgeuze and Antares science fiction comic series by Leo, a Brazilian artist. You can find some on the english translations of Leo's Aldebaran series here. The creatures of these series are thought through quite well, being quite weird and alien indeed.
A third thing to look at, when you're able, is Falling Skies, a science fiction series now in its fourth season, produced by Steven Spielberg. It features quite a few special effects and weird creatures.
Good luck imagining! - oh... and if you can stand Niven, read the Ringworld series!