In case you've ever wanted to transform your mug into an Aperture Science Portable Quantum Tunneling Device, crafter Two Horns United has built one that fits right on your chin. I imagine it's incredibly useful if you're the kind of clumsy individual who frequently finds yourself plummeting off cliffs face-first. Wear it to the Portal golf course for extra thematic resonance.
DISCUSSION
"...frequently finds yourself plummeting off cliffs face-first." That'd only be useful if you'd already opened a portal somewhere else. (Or, of course, is quick enough to open two portals side-by-side.) Personally, I'd prefer the long-fall boots if I were that clumsy.