You may not know the name, but you have definitely seen his work somewhere. Whether it be in comics, movie posters, album covers, movies, or book covers.
He was a major influence in my fantasy artwork and an influence to many others, I'm sure. I remember seeing the movie "Fire and Ice" when I was young and being fascinated by the images - not knowing who the conceptual artist was behind them. Frazetta is definitely one of the reasons I migrated towards illustration. His artwork is powerful and full of tension. He was able to capture a scene at just the right moment without making it look static.
I looked through a lot of his images to find a good one to place here and couldn't decide so I picked one at random (still a favourite). If you'd like to see an extensive online collection you can go here.