A good and sharp seam ripper and a set of squaring rulers. (I got mine with the 50% off coupon at Joann) 6 1/2" - 9 1/2" - 12 1/2" are great sizes. I also have the 15" square but find it easier to use rulers closer to the block size.
Recently I bought the wonder clips (both small and large) which are great for binding.