I've knitted with 100% hemp, but only to make belts and bags--things that I want to be rough and firm and non-stretchy. If you're set on a hemp hoodie, like other folks, I suggest a hemp/wool blend. Check out the Lana Knits (a yarn brand) web site to see what hemp and hemp blends are available. They have great stuff! If you're really curious about hemp, they even have a hemp yarn sampler for $25, which gives you a variety of hemp and hemp-blend yarns to play with.
I also agree with the folks who suggest bamboo yarn. I've made a sweater with Bam Boo by Classic Elite, and it's soft and silky and wonderful! This stuff feels a lot like rayon, but it's less icky if you sweat on it. Similar interesting, soft yarns on the market include soy fiber yarn, and even corn yarn!
Lana Knits (hemp yarn): www.hempforknitting.com
Soy and corn yarn: www.soysilk.com
Bamboo yarn: www.classiceliteyarns.com