There isn't much science with this gadget: it's a slim, rubber spatula with a wooden handle and a rounded edge ($6.99 at Macy's, not available online). Only about a half inch wide, this jar spatula fits into any jar or crannied container to scoop up its most wedged-in, hard to reach remains. I love having it around for those expensive jars of jam or barely-large-enough-for-one-more-serving marinades, when it helps me make the most of what little is left and ensure none of my precious goods go to waste.
The handle is sturdy and comfortable to hold, while the rubber head is thick enough to successfully scrape sticky goods rather than bending and caving. The proportions are well thought out for jars of different sizes, as is the overall shape. My main gripe is that after several washes, the rubber head sometimes slips off the wooden handle—while I can fix it fairly easily, I'd hate to lose it at any other moment.
For the price, the product is surprisingly versatile beyond jars: It's great for delicately lifting crepes and features a flat side that comes closer to an offset spatula than any other cousin on the market. For a few extra tasks and durability, go with a silicone model like this Oxo one—the heat resistant material adds functionality and prevents melting or discoloring.