Hitech or Singh-Ray Grad ND filters?
I can't speak for the Singh-Ray grads (although I hear they're excellent) but the Hitech are quite good for what they cost. The Grad ND's are vastly superior to the Cokin versions. I also own the Pro Stop 10 stop ND and although there is definite colour cast, it's easy to fix in post.
Id highly recommend Hitech for the value if what you get. I know I personally plan to switch all my drop in filters to Hitech.