RedPlum: Sometimes you'll find RedPlum inserts in your Sunday newspapers, but if you don't buy the Sunday papers or you'd like additional RedPlum coupons, you can sign up for The Red Plum Mail package. Be sure you select "add to list."
Bounce: When Bounce has coupons available by snail mail, you'll find the link on their "promotions" page.
P&G Brand Sampler: You'll have to register to get the free Home Made Simple coupon booklet via snail mail. Registration is free, and as a P&G Everyday Solutions member myself, I can honestly tell you that taking the time to register is well worth it. Their coupon booklets are usually filled with great savings!
Minute Maid Coupons by Snail Mail: Residents from the United States and Canada can enter their details in the form on the linked page to request money-saving coupons from Minute Maid.
Tide: When Tide has coupons available, you'll find them on their "coupons" page.
Another Way to Obtain Coupons by Snail Mail
Google the manufacture's name to see if they have a website, and if they do, contact them through their website asking if they could send you coupons by snail mail. Before asking for the coupons, state that you've tried and liked their product(s), or that you'd like to try their product(s). You can do this for restaurants as well.
I'll add to this list when I discover new resources, so check back often, bookmark this page or follow me on Twitter for updates. Also, if you know of any other websites where frugal shoppers can sign up/register to get coupons by snail mail, let me know by leaving a comment, and I may even add it to this list giving you credit for the referral. 🙂