There are plenty of gadget/cutters/rulers that I could mention.
Lord knows, I have more than my fair share!
However, the one purchase that has totally revolutionized my quilting is a book.
Crazy Shortcut Quilts by Marguerita McManus & Sarah Raffuse
I've not done a crazy quilt but their QAYG method has made quilting a real joy for me.
I just could not stuff anything larger than a 60"x60" lap quilt (on a good day) into the throat of my sewing machine without wanting to pull my hair out.