Optionify:More Product Options , 90 reviews
It doesn't work on Shoptimized Theme. But for standard (old Themes) is good. A lot of features.
Did not work for Jumpstart theme (a shopify theme) and I felt it was a bit of a hack job, with a product that shows up in your store but can't be deleted. However, maybe it works well for other themes. Wasn't right for me unfortunately.