I know that Samson is a favorite, and rightfully so, I love it too. I've been using ATWOOD Rope utility cord as guylines after stumbling onto them at a local surplus/camp/pack store. The utility cord is super-cheap and holds prussics like iron. It's not a limp cord, either. All manufactured in the USA, so that's good.
I have an email into them asking if they make an amsteel competitor. Anybody else have experience with their products?