As a lot of us are finding that we need to order some of our specialty baking items and products because they are not sold locally, I am starting this thread so we can list those places. I live in a rural area now, so I do even more online ordering than previously: Here are mine:

King Arthur Flour--For me their Bakers Bucks program makes sense, but it probably doesn't for everyone. They do have various specials, so it helps to be on the email list. It's where I go for special clear flour, regular pastry flour, Vermont Cheese powder, espresso powder, durum flour, semolina, medium rye flour, and yeast. I also use a lot of their parchment.
https://www.kingarthurflour.com/shop-home/index.html?go=Home

Penzey's Spices--If you are on their email list, you will be notified of specials, as well as freebies with an order. Shipping is free over $30, but they run a lot of $20 shipping specials. (See S. Wirth's note about current vanilla special.)
https://www.penzeys.com/

Bob's Red Mill--Here again, it pays to get their emails, as they have sales throughout the year. Shipping is free for orders over $50, but that does not include 25 pound bags. It does include cases with multiple bags, and there is a discount for cases. In addition to flours, they carry whole grains I've not see elsewhere, coconut, dates, beans. Their unbleached cake flour performs well, as does their Artisan bread flour. I like their vegi-soup mix of dried split peas, lentils, and barley. I also like their long-grain brown rice, golden couscous, and oats of all kind.
https://www.bobsredmill.com/

Kitchen Krafts--the only place I've found online for buying ClearJel (not the instant stuff), and also some canning tools. I've not found their shipping prices to be particularly good. They do have various specials, so it pays to be on their email list. Their return policy is cumbersome.
https://www.kitchenkrafts.com/

Fancy Flours--a good place for some specialty cookie cutters, springerle molds, wafer paper, and some specialty flavorings (where I found strawberry a few years ago). The topper for our wedding cake came from here. Their emails sometimes have specials.
http://www.fancyflours.com/