A few years ago we started bringing a spare carry-on bag with us on international trips to stuff with food from wherever we were. As the most recent trips have been to Italy, that bag comes back stuffed with oils, vinegars, pastas and hard cheeses not found around Pittsburgh. We've learned to pack light enough where both of us have extra room for stuff and still not pay a luggage premium.
From Austrialia, of all things, we brought back Tim-Tams, which are frankly, the most ridiculously addicting cookie on the planet. We brought back six boxes for friends and ate a box and a half on the plane. We also brought back some Indonesian spices and cooking ingredients we'd never seen here. From Mexico we've taken back dried huitlacoche, fresh corn tortillas and some bean varieties that aren't the same here.
Carrying back regional wine, beer (VB!) or booze goes without saying (where legal, of course). So does a great parmigiana-reggiano or other dry aged cheese. Aged vinegars, even "00" flour or something simple that can be had for pennies but costs a fortune here. Surprisingly, there are some great dried pastas that are difficult to get in the US without paying through the nose (strangozzi comes to mind - just had some tonight with a simple toss of olive oil flavored with the Spice Man's (Firenze) 'Mauro' blend and some garlic).
So bringing the empty bag and stuffing it with comestibles has been worth it - sometimes it weighs a good 20lbs or more. And if you don't eat everything right away, a few months later you can have a feast and remember your trip fondly.
But back to the question at hand... What foodstuff do you toss in your bag when heading home that you can't get in the US, or that is of better quality (or value) than what you can get in the US?