Tri-State's been a staple of Moscow shopping since 1946, so I have some fond memories from shopping there. You can find outdoor and casual clothes, shoes, camping gear, guns, rifles and ammo, grills, as well as an insane variety of mugs from small to huge, and much, much more.
The employees are always friendly and laid-back, ready to help.
Since I only get back to Moscow every few years, I like the selection of Idaho souvenirs available (though I wish they had a couple more shelves) where you can buy Idaho-themed chocolate, caramel corn, huckleberry jam and more. There are also soup and chili mixes from Washington and other neighboring areas that make great gifts. They are expensive, though, but that's something you can say about everything these days...
I also appreciate the selection of BBQ spices and rubs they have available, so I always pick up a couple before returning to Japan as the selection there is woefully small. The rather extensive selection of kitchen goods is also wonderful!
The prices are not as cheap as, say, Wal-mart or Ross or something, but Tri-State offers higher-quality products and items you can't find in other places. So, I recommend checking them out! In a world of box-stores that all seem to blend together (ie. Costco, Walmart, etc) Tri-State offers a unique an fun experience. Oh, and not to mention Ace Hardware that is connected to the store as well.
IMO, those are some of the reasons Tri-State has survived so long.
One side note--when I was a kid in the late 80s, they used to give out free mini candy canes during Christmas. My sister and I took WAY too many and probably caused a few headaches for the employees... sorry about that!